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What is the difference between byte and byte array?

What is the difference between byte and byte array?
byte[] array1 ={1,0,0,0}
Does this means that the array 1 is having byte value of 1000?
How can I differentiate when to use byte and byte array?

A byte is (in the case of c#) an unsigned integer composed of 8 bits, so: an integer in the range [0,255]; a byte[] is a fixed size chunk of byte values, in this case 4 values, with initial values (sequentially) one, zero, zero, zero. This is not the same as a value of 1000 - it is 4 discreet values. You could coerce a byte[] payload to an integer, but what value that means is ambiguous:
we could treat it as a raw big-endian 32-bit integer
we could treat it as a raw little-endian 32-bit integer
we could treat the 4 elements as decimal digits
we could treat the 4 elements as ASCII characters that might represent decimal digits
or the same with a non-ASCII encoding, for example UTF-16 (big or little endian), UTF-32, etc
etc
As for when to use each: are you talking about one value, or multiple values? note that byte[] is typically used when talking about binary payloads such as file/network contents, although you can use byte[] for more specific scenarios unrelated to this.

byte is a variable that will be between 0 and 255 . byte array is an array that contains byte values ( value 0 to 255)

An array is a structure containing multiple values of the same type. A byte array therefore contains multiple bytes. Your array four bytes. The first one is 1, second one 0, third one 0, and so on. The array does have a value of {1,0,0,0}, or [1,0,0,0], and when you call its ToString() method, you get "System.Byte[]".

What exactly is byte[] in C#?

In C#, byte is the data type for 8-bit unsigned integers, so a byte[] should be an array of integers who are between 0 and 255, just like an char[] is an array of characters.
But most of time when I encounter byte[], I see byte[] is used as a contiguous chunk of memory for storing raw representation of data.
How do these two relate to each other?
thanks

Well, a byte as datatype is exactly what you already said, an unsigned integer between 0 and 255. Furthermore this type needs exactly - believe it or not - one byte in your memory, thus also the name. This is why most readers that read byte per byte store those information in a structure that fits exactly the size of a byte - the byte-datatype.

Converting int to hex but not string

Now I know that converting a int to hex is simple but I have an issue here.
I have an int that I want to convert it to hex and then add another hex to it.
The simple solution is int.Tostring("X") but after my int is turned to hex, it is also turned to string so I can't add anything to it until it is turned back to int again.
So my question is; is there a way to turn a int to hex and avoid having turned it to string as well. I mean a quick way such as int.Tostring("X") but without the int being turned to string.

I mean a quick way such as int.Tostring("X") but without the int being
turned to string.
No.
Look at this way. What is the difference between those?
var i = 10;
var i = 0xA;
As a value, they are exactly same. As a representation, first one is decimal notation and the second one is hexadecimal notation. The X you use hexadecimal format specifier which generates hexadecimal notation of that numeric value.
Be aware, you can parse this hexadecimal notation string to integer anytime you want.
C# convert integer to hex and back again

There is no need to convert. Number ten is ten, write it in binary or hex, yes their representation will differ depending in which base you write them but value is same. So just add another integer to your integer - and convert the final result to hex string when you need it.
Take example. Assume you have
int x = 10 + 10; // answer is 20 or 0x14 in Hex.
Now, if you added
int x = 0x0A + 0x0A; // x == 0x14
Result would still be 0x14. See?
Numeric 10 and 0x0A have same value just they are written in different base.
Hexadecimal string although is a different beast.
In above case that could be "0x14".
For computer this would be stored as: '0', 'x', '1', '4' - four separate characters (or bytes representing these characters in some encoding). While in case with integers, it is stored as single integer (encoded in binary form).

I guess you missing the point what is HEX and what is INT. They both represent an numbers. 1, 2, 3, 4, etc.. numbers. There's a way to look at numbers: as natural numbers: INT and hexadecimal - but at the end those are same numbers. For example if you have number: 5 + 5 = 10 (int) and A (as hex) but it the same number. Just view on them are different

Hex is just a way to represent number. The same statment is true for decimal number system and binary although with exception of some custom made numbers (BigNums etd) everything will be stored as binary as long as its integer (by integer i mean not floating point number). What would you really like to do is probably performing calculations on integers and then printing them as a Hex which have been already described in this topic C# convert integer to hex and back again

The short answer: no, and there is no need.
The integer One Hundred and seventy nine (179) is B3 in hex, 179 in base-10, 10110011 in base-2 and 20122 in base-3. The base of the number doesn't change the value of it. B3, 17, 10110011, and 20122 are all the same number, they are just represented different. So it doesn't matter what base they are in as long as you do you mathematical operations on numbers in the same base it doesn't matter what the base is.
So in your case with Hex numbers, they can contain characters such as 'A','B', 'C', and so on. So when you get a value in hex if it is a number that will contain a letter in its hex representation it will have to be a string as letters are not ints. To do what you want, it would be best to convert both numbers to regular ints and then do math and convert to Hex after. The reason for this is that if you want to be able to add (or whatever operation) with them looking like hex you are going to to need to change the behavior of the desired operator on string which is a hassle.

Converting Int32 to 24-bit signed integer

I have a need to convert an Int32 value to a 3-byte (24-bit) integer. Endianness remains the same (little), but I cannot figure out how to move the sign appropriately. The values are already constrained to the proper range, I just can't figure out how to convert 4 bytes to 3. Using C# 4.0. This is for hardware integration, so I have to have 24-bit values, cannot use 32 bit.

If you want to do that conversion, just remove the top byte of the four-byte number. Two's complement representation will take care of the sign correctly. If you want to keep the 24-bit number in an Int32 variable, you can use v & 0xFFFFFF to get just the lower 24 bits. I saw your comment about the byte array: if you have space in the array, write all four bytes of the number and just send the first three; that is specific to little-endian systems, though.

Found this: http://bytes.com/topic/c-sharp/answers/238589-int-byte
int myInt = 800;
byte[] myByteArray = System.BitConverter.GetBytes(myInt);
sounds like you just need to get the last 3 elements of the array.
EDIT:
as Jeremiah pointed out, you'd need to do something like
int myInt = 800;
byte[] myByteArray = System.BitConverter.GetBytes(myInt);
if (BitConverter.IsLittleEndian) {
// get the first 3 elements
} else {
// get the last 3 elements
}

If byte is 8 bit integer then how can we set it to 255?

The byte keyword denotes an integral
type that stores values as indicated
in the following table. It's an Unsigned 8-bit integer.
If it's only 8 bits then how can we assign it to equal 255?
byte myByte = 255;
I thought 8 bits was the same thing as just one character?

There are 256 different configuration of bits in a byte
0000 0000
0000 0001
0000 0010
...
1111 1111
So can assign a byte a value in the 0-255 range

Characters are described (in a basic sense) by a numeric representation that fits inside an 8 bit structure. If you look at the ASCII Codes for ascii characters, you'll see that they're related to numbers.
The integer count a bit sequence can represent is generated by the formula 2^n - 1 (as partially described above by #Marc Gravell). So an 8 bit structure can hold 256 values including 0 (also note TCPIP numbers are 4 separate sequences of 8 bit structures). If this was a signed integer, the first bit would be a flag for the sign and the remaining 7 would indicate the value, so while it would still hold 256 values, but the maximum and minimum would be determined by the 7 trailing bits (so 2^7 - 1 = 127).
When you get into Unicode characters and "high ascii" characters, the encoding requires more than an 8 bit structure. So in your example, if you were to assign a byte a value of 76, a lookup table could be consulted to derive the ascii character v.

11111111 (8 on bits) is 255: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1
Perhaps you're confusing this with 256, which is 2^8?

8 bits (unsigned) is 0 thru 255, or (2^8)-1.
It sounds like you are confusing integer vs text representations of data.

i thought 8 bits was the same thing as
just one character?
I think you're confusing the number 255 with the string "255."
Think about it this way: if computers stored numbers internally using characters, how would it store those characters? Using bits, right?
So in this hypothetical scenario, a computer would use bits to represent characters which it then in turn used to represent numbers. Aside from being horrendous from an efficiency standpoint, this is just redundant. Bits can represent numbers directly.

255 = 2^8 − 1 = FF[hex] = 11111111[bin]

range of values for unsigned 8 bits is 0 to 255. so this is perfectly valid
8 bits is not the same as one character in c#. In c# character is 16 bits. ANd even if character is 8 bits it has no relevance to the main question

I think you're confusing character encoding with the actual integral value stored in the variable.
A 8 bit value can have 255 configurations as answered by Arkain
Optionally, in ASCII, each of those configuration represent a different ASCII character
So, basically it depends how you interpret the value, as a integer value or as a character
ASCII Table
Wikipedia on ASCII

Sure, a bit late to answer, but for those who get this in a google search, here we go...
Like others have said, a character is definitely different to an integer. Whether it's 8-bits or not is irrelevant, but I can help by simply stating how each one works:
for an 8-bit integer, a value range between 0 and 255 is possible (or -127..127 if it's signed, and in this case, the first bit decides the polarity)
for an 8-bit character, it will most likely be an ASCII character, of which is usually referenced by an index specified with a hexadecimal value, e.g. FF or 0A. Because computers back in the day were only 8-bit, the result was a 16x16 table i.e. 256 possible characters in the ASCII character set.
Either way, if the byte is 8 bits long, then both an ASCII address or an 8-bit integer will fit in the variable's data. I would recommend using a different more dedicated data type though for simplicity. (e.g. char for ASCII or raw data, int for integers of any bit length, usually 32-bit)