Number systems are used to represent numeric data and there are three main types: decimal, binary, and hexadecimal. The decimal system has a base of 10 and uses the digits 0-9. The binary system has a base of 2 and uses only the digits 0 and 1. The hexadecimal system has a base of 16 and uses the digits 0-9 and A-F, with A representing 10, B as 11, and so on up to F for 15.