Hexadecimal notation (or hex for short) is a numeric system where the base number is 16 rather than 10 in common math.  Hex adds the symbols of A-F beyond the counting symbols of 0-9 to it's digit system.  This means that 10 displayed in hexadecimal is actually equal to 16. 

Hex digits are 4 bit values.  Two hex digits placed next to each other represent an 8 bit value (0-255).