Hexadecimal

A numbering system that uses 16 different digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.

Because each hexadecimal digit has more different values than our familiar decimal number system (where there are only 10 different digits, 0 through 9), hexadecimal number can represent large values using fewer digits. 

For example, the decimal number 4,294,967,295 is equal to the hexadecimal number FFFFFFFF (or FFFF FFFF, since spaces are often inserted every four hexadecimal digits to make them more readable). 

A single hexadecimal digit can represent all the possible values of a group of four bits. The possible values of a single byte range from 0 to 255 (in the decimal system), or 0 to FF (in the hexadecimal system).

Note: The word "Hexadecimal" is often shorted to just "Hex".