While the decimal numbering system allows any digit in a number to have any of ten values (0-9), digits in hexadecimal numbers can have 16 (0-F), digits in octal numbers only eight (0-7) and ...

You will see Hex (Hexadecimal), Dec (Decimal), Oct (Octal) and Bin (Binary) radio buttons. These are used for converting numbers between these numbering systems.