Hexadecimal (often abbreviated hex) is a base 16 number system, which consists of symbols 0-9 and A-F. It is a useful system in computers because there is an easy mapping from 4 bits to a single hex digit. Thus one can represent every byte as a two consecutive hexadecimal digits.
0001 = 1 0010 = 2 0011 = 3 0100 = 4 ... 1001 = 9 1010 = A = 10 1011 = B = 11 ... 1111 = F = 15
So the the binary representation for 79 (0100 1111) can be written as 4F.
There are many ways to denote hexadecimal numbers. One (derived from C) is to start with '0x'. Pascal programmers would indicate hex by an appended 'h'. Sometimes a prefixed dollar symbol signals a hexadecimal number. When numbers of various bases are used together, the base is often noted in a subscript of the number.
The word "hexadecimal" is strange in that "hexa" is derived from Greek and "decimal" is derived from Latin. The original term was the fully-Latin "sexidecimal", but that was changed because some people thought it to be too racy.