Your IP address is:
How do we know ?
If you are interested in how we know your IP address, then with every request that is made to a web server, it gets what address is connecting and forward it to the application. Your browser also sends other useful information like its type/version (called USER-AGENT), the languages that you speak, etc, and obviously your cookies.
Does it identify me personnaly ?
Not exactly. An IP address on the world wide web (not on the subnet) identifies a connection to the internet. For example in a cafe with free wi-fi, everybody has the same IP address. But using a mix of your IP address and other apparently unpersonnal info can make it potentially possible to identify you personnaly with a little help of good old probablility.
What if I don't want websites to know my IP ?
It is quite useful for websites to know your IP address because they can track malicious activities and find out who is to blame and black list the IP address if necessary. But on the other hand it can be abused and help follow your internet activities.