Every computer on a typical network is assigned a unique series of numbers specific to that network that identifies it separately from all other computers and devices (printers, routers, switches) on that same network. This is also true of anything connected to Internet as well. This series of numbers is referred to as the IP address. IP stands for Internet Protocol.
Every computer and server on the Internet can be reached through its IP address, whether you are trying to get to a website hosted on it, or sending email to a mailbox stored on it, or even connecting to it to work interactively with a remote desktop or application.
I can hear your first comment already: "But I've never typed in a number to get to a website.", "I type in a domain name, not a number, to send email to somebody!". Because people tend to remember names much easier than numbers, another system was setup to make using these numbers, DNS. DNS acts much like a phone book for computers, matching domain names to IP addresses the way a phone book matches people's names to phone numbers. This system works so smoothly that for most scenarios you don't even need to know that IP addresses exist.
However, many home networks don't get a DNS entry setup, as their IP address changes on a regular basis. This isn't normally a bad thing, but it does make it trickier to find out what IP address you are using to get to the Internet. In the unusual case where you need to find out what your IP address is, there are actually a few websites you can visit that will tell you what your IP address is for you.
Www.whatismyip.com gives you your IP address and also provides more information about IP addresses in general and DNS.
Www.ipaddress.com will show you your IP address and some more information about your connect as well.
It isn't often that you need to worry about what IP address you are using to connect to the Internet, but when you need it, these websites make it simple to find out.