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WPF: Create a UserControl or a CustomControl?

I'm not very clear of when to use a CustomControl and when to use an UserControl. I basically know what CustomControl allow(more customization when using with template).
I want to make a "File browse" control in WPF(TextBlock that displays the current path + Button that trigger a "Open File Dialog").
I'm not sure, because at some places I find that they say this should always "replace" a WPF control. I was more thinking that a a CustomControl was more like a way to display and edit one new semantic value (in my case, a "File(path)").
So, if we don't take into account which one is easier to do, what would be the more adequate between CustomControl and UserControl for the "FileBrowse" control that I'm speaking about?
Thank you

Custom Control:
A loosely coupled control w.r.t code and UI
Derives from Control
Defines UI in a ResourceDictionary
UI can be changed in different projects
Has full toolbox support
Defines a single control
More flexible
User Control :
A tightly coupled control w.r.t code and UI
Derives from UserControl
Defines UI as normal XAML
UI is fixed and can't have different looks in different project
Can't be added to the toolbox
Defines a set of controls
Not very flexible like a Custom Control
When we talk about differences, it is more important to emphasis on the context when to use what:
When you have a rapid and fixed content in your UI, use UserControl.
When you want to separate some basic functionality of your main view to some smaller pieces with reusability, use UserControl.
When you want to use your control in different projects and each project may want to change the look, use CustomControl.
When you want to implement some additional functionality for a control, create a CustomControl derived from the base control.
When you want to apply themes to your controls, use CustomControl.
When you want to add toolbox support for your control, so that your user will be able to do drag and drop to the designer, use CustomControl.

I think a UserControl is the one to choose, for it is used for a kind of "assembly of existing controls". In your case a button and a file open dialog. It will then get a specific look and feel (for example the default look of a button and the default look of a file open dialog).
The CustomControl is kind of the other way round. It has no look and feel by itself. It is completely abstract concerning layout. The layout comes into play, wwhen assigning a style to it.

In general a custom control extends an existing control while a user control creates a new control type from a collection of existing controls. I would say that a user control is better suited, based on the information you've given.

C# WinForms add controls programmatically

Can someone suggest the best way to achieve my goal?
So, I have a form with three buttons. What I want is, depending on what button is pressed on panel should be shown different controls (user control). I made this in a simple way: all are added from the beginning, I just make change to the visibility. But what would be nice is, if someone can suggest a more appropriate way, because there is no need to have objects created from beginning.

You can always create the appropriate UserControl, and add it to the Panel.Controls at runtime. This will allow you to create the control(s) as needed, instead of on initialization of your Form.

I would indeed create the controls at design time - if there's advantage no to dynamically create them. Why complicate matters?
If there are a number of controls I would put them all in a panel (within the panel you've already mentioned) so you're only changing the visibility of a single control (the panel) rather than each one within it.
When you press the appropriate button show the appropriate panel (and remember to hide the others in case you've previously shown them)

Create controls dynamically or create controls in a side form? C# winforms

I have a form full of controls, and there is no room for other controls. On the bottom of the form I have a panel with some controls on it.
My goal is that when a certain button is clicked, the original panel on the bottom will be replaced with another panel that contains controls which could be created before the program starts, meaning these controls in the panel do not need to be created dynamically. The replace action would be executed by setting each panel's visible field to it's matched value.
I have thought of two ways of doing this - either creating the new panel (and it's controls) dynamically and adding it to the form instead of the original, or creating the new panel in another form and when the relevant button is clicked the panel being taken from that form and added to the required form (by creating an instance of the new form and making it's panel's modifier public). The "side form"'s purpose is only to create that panel, it has no functionality of it's own.
The advantages of creating the new panel dynamically:
There is no need to create a zero-functionality form.
The advantages of creating the new panel in a side form:
It's very clear which controls are added to the new panel and their positions.
It's very easy to set the location and other fields of the controls in the new panel.
Which way is better?
Thanks!

Have you considered TabControl? That seems a good fit for your needs. Other controls I can think of are StackPanel (Can be fairly easily done for Windows Forms) or OutlookBar like control (again a user control).
Simplest and quickest way seems to be TabControl.
Edit:
SideForm is a different windows form I suppose. So if you are thinking to make controls public and then change their visibility etc, please don't. Use delegates to handle SideForm's events in MainForm.
As you mentioned, there is no room for more controls, I would suggest more screens rather than just one. Having said that I do not know much about your current UI design and functionality so it's up to you.
I would say having the controls hidden and just playing with the Visibility is fine. This means that you do not have to worry about positioning of controls, anchoring and docking at runtime. The problem could well be loading of form. Having huge number of controls having a lot of data associated with them may slow things down.

IMO the best way would be to utilise user controls for this purpose. Simply create one user control per panel you wish to show/hide and place your controls inside. This way you will have both: the designer and the "extra form" you wanted.

difference between Panel and UserControl c#

Could someone please tell me the differences between using a Form, Panel or a UserControl.

A form is a control and a container for other controls. A form is the base unit of a windows application.
A panel is a control and a container for other controls.
A usercontrol is a user defined control.
See:
Windows Forms
Windows Forms Controls
Windows Forms Overview
In Windows Forms, a form is a visual
surface on which you display
information to the user. You
ordinarily build Windows Forms
applications by adding controls to
forms and developing responses to user
actions, such as mouse clicks or key
presses. A control is a discrete user
interface (UI) element that displays
data or accepts data input.
When a user does something to your
form or one of its controls, the
action generates an event. Your
application reacts to these events by
using code, and processes the events
when they occur. For more information,
see Creating Event Handlers in Windows
Forms.

According to MSDN the Panel class is "Used to group collections of controls", while the User Control "Provides an empty control that can be used to create other controls".
You are right: this doesn't help you a lot to decide whether you should use a Panel or a User Control.
One of the differences is that a Panel is a ScrollableControl, while a UserControl is a ContainerControl (which is also a ScrollableControl). So if you want ContainerControl functionality, consider to use a UserControl.
You'll probably don't know what a ContainerControl does, so what you can't do with a Panel, hence the following might be more useful:
In object oriented programming, and so also in Winforms, whenever you want a class that behaves like another class, but only slightly different, you consider to derive from the other class.
So if you want a button that changes color when pressed, and returns to its original color when pressed again, (like an on-off button), you might consider to derive from class Button, or maybe from class CheckBox-in-the-shape-of-a-button.
By making it a separate class, you can reuse the code in similar situations. Whenever you will only use it once, then usually we won't bother to make it a special class. We will usually not make a special class for "The Select button in my form, which does ... when clicked", but if you will use this button in ten different forms, then it is probably wiser to create a SelectButton class.
Similar, if you have a group of controls, with some behaviour, and you plan to use that in different forms, consider to create a User Control, where you put this behaviour. The nice thing is that the code of this behaviour is hidden inside the control. Users of your UserControl only have to know what it does, not how this is done. You might even want to hide how this is done, so users (= code, not operators) can't access it
A panel is more or less like a GroupBox without a surrounding rectangle: consider to use it instead of a User Control if you will be using it only inside this Form. Similar to how you would us a "Button that does ... when clicked": because you use it only here, you don't derive from it.
I seldom use a Panel. The derived classes: TabPage, SplitterPanel, ... are more likely to be used only in this form.
Whenever I need combinations of several controls, especially if they interact with each other. For instance, if you have a text box and a label that describes what is in the textbox and an OK button that processes the text in the text box. In that case I usually make it a UserControl.
I could have derived from a Panel and add a Label, TextBox and Button, but then users could mess up with my Panel by adding other items, or calling Panel functions that would mess with my functionality.
Come to think of it: using a class derived from a Panel vs using a UserControl is similar to deriving vs aggregation / composition: If you aggregate, you can limit access to functionality, if you derive, users can access all parent functionality.
So if you only want limited functionality: show / no show, maybe size and position, background, but not much more: consider to create a UserControl. If you want the possibility to change the behaviour, consider to use a Panel, especially if you will use it in only one form.

How to implement a-form-inside-a-form with runtime embedded forms switching?

I need to implement TabControl-like behaviour with manual (on event, on a button click for example) pages switching and having all pages designed and implemented as separate forms. A form to be incorporated (as a panel control) inside main form and replaced by another form as needed.
How to achieve this?
PS: The reason why I don't want to use TabControl instead is because there are going to be too many tabs - I'd prefer to present the list of them as a TreeView and instantiate on demand. The another reason comes from another project of mine - there I am going to implement plug-ins, where a panel inside main window will be provided by a class loaded dynamically and will be runtime-switchable.

I need to implement TabControl-like behaviour with manual (on event, on a button click for example) pages switching and having all pages designed and implemented as separate forms
May I ask why this is a requirement? It seems like the logical approach would be to create a set of UserControls. You can place a UserControl in a form, and you can place a UserControl in a tab. You get modularity without the headache of implementing a very odd requirement which is a use case that the API developers obviously did not think was valid. I just can't think of a good reason to take the route you have suggested.

I did similar thing once, and for that reason, I have ReplaceControl method, which I paste below:
static public void ReplaceControl(Control ToReplace, Form ReplaceWith) {
ReplaceWith.TopLevel=false;
ReplaceWith.FormBorderStyle=FormBorderStyle.None;
ReplaceWith.Show();
ReplaceWith.Anchor=ToReplace.Anchor;
ReplaceWith.Dock=ToReplace.Dock;
ReplaceWith.Font=ToReplace.Font;
ReplaceWith.Size=ToReplace.Size;
ReplaceWith.Location=ToReplace.Location;
ToReplace.Parent.Controls.Add(ReplaceWith);
ToReplace.Visible=false;
}
Only thing left to do is to create some control manually on the form, as the placeholder for your Form. Use label, for example.

You could do this with an MDIForm as the main form, and then plain-old Forms as the separate forms. Or you could encapsulate each element's functionality as a UserControl which you can then swap out on your form in code.
The advantage of encapsulating your UI elements as UserControls is that if, for whatever reason, you need them to become forms in your application, you can just drop the UserControl on a form.
Update: Since you want to use a TreeView to select what the user is looking at, you definitely want to do this as a bunch of UserControls. The layout is simple: TreeView on the left, and whichever control is active on the right.
There's no need to justify not using a TabControl - tabs are the worst UI element in history.