Computers work with binary numbers (base 2) while people usually work with decimal (base 10). Binary numbers are long strings of ones and zeros that are difficult for people to read and so to make it easier for people to read the numbers the computers are working with a different number base that is directly related to the binary number is generally used. Originally octal (base 8) where three binary digits are combined to display as a value between 0 and 7 was used however as computer bytes standardised on a length of eight binary digits (or bits) hexadecimal (base 16) using A through F to represent the values 10 through 15 as a single digit has become the alternative of choice as it easily allows the content of a byte to be displayed as a two digit hexadecimal number between 00 and FF.
As an example, colours used on a web page have one byte allocated for each of the red, green, and blue components with each being able to therefore have any value between 00000000 and 11111111 (binary) or 00 and FF (hexadecimal) or 0 and 255 (decimal). We can specify the colour white in a stylesheet as either '#FFFFFF' using hexadecimal or rgb(255,255,255) using decimal.
This article written by Stephen Chapman, Felgall Pty Ltd.