A number system is defined using a base, where a base is another natural number. We use a base of 10 (called the decimal system). In computers, the base is 2 (binary). Octal (base - 8) and Hexadecimal (base - 16) are also popular in computer science. It is often forgotten that we use the hexagesimal system (base - 60) many times daily, whenever we look at our watch. You can actually have infinite number of number-systems, as long as you can justify why you would need them. The general rule is that, you start counting from some start point (say 0) and go upto the base-1 number (such as 7 in the case of octal system) and then start from 0 once again.