Human beings have a total of 10 symbols to represent numbers: from 0 to 9. By combining them, you can represent any number – 13, 648, 2015, etc.. In the same way just about anything in the universe can be represented with just 1’s and 0’s. You just need a system to combine them.
Imagine four switches that can either be turned on or off. Each switch has its own value, ’8′, ’4′, ’2′ and ’1′. When they are all turned off that represents ’0′ – 0000. To represent the number one you simply turn on the ’1′ switch – 0001. To represent the number two, you turn off the ’1′ switch and turn on the ’2′ switch – 0010. Now think carefully and try to represent the number three. You turn on both the ’2′ and the ’1′ switch – 0011. Because 2 plus 1 equals 3.
Try to get the number seven. Turn on switches ’4′, ’2′, and ’1′ – 0111. Try the number 11. Turn on switches ’8′, ’2′ and ’1′ – 1011.

Look at the following table to understand how it works: