The hexadecimal system is also called Base 16. It is based on 16 in the same way that the decimal system is based on 10:
1 2 3 4 5 6 7 8 9 A B C D E F 0
Examples:
Hexadecimal = Decimal
1 = 1
A = 10
F = 15
10 = 16
64 = 100
FF = 255
100 = 256
3E8 = 1000
FFF = 4095
1000 = 4096
2710 = 10000
FFFF = 65535
10000 = 65536
See:
Decimal Numbers