The base we are the most familiar with is the decimal system or base 10. What this means is that, each 'slot' has 10 possibilities before a second digit is needed (0,1,2,3,4,5,6,7,8,9). For example we can have the digits 0-9 before needing to add a second digit. 11 requires 2 slots or places. In binary, there are only two possibilities (0 or 1) before adding another digit. So, zero is 0 and one is 1, but two is written as 10, three as 11, and four as 100
For more infomation on base numbers see the Radix