The Binary or base-2 counting system is a system of numbers using only 0 and 1.

For example the binary number $110101_2$

 $2^5=32$ $2^4=16$ $2^3=8$ $2^2=4$ $2^1=2$ $2^0=1$ 1 1 0 1 0 1

represents the number 32 + 16 + 4 + 1 = 53

Because of the simplicity of the binary counting system and normally electromagnetic storage devices only have the facility to store information in one of two states i.e. on or off, computers make great use the base-2 counting system.