You can google "Internet Protocol" to find out all about IP. It is a number in groups of three digits sepasrated by dots. This number is what the URL gets translated into so the network routes your request tot he correct machine in the network or internatwork.
If instead of a URL you were to enter Microsoft's real Web address is 207.46.230.229 into your browser, it used to take you to Microsoft.com.
An explanation can be found here and many more can be found here.