What's an IP Address?

An IP Address can be compared to an address used for sending and recieving packages.
An IP address is split into 4 sections.
Like your the address you write on a letter: you put the name, the street address, the city, state, postal code, and the country.
The subnet mask makes a division in the address, defining which part of the address corresponds to the network and the other to the host computer.

You could as well think of it as a phone number.
You have a specific number people use to call you.
As well you can use specific phone numbers to reach other people.
IP addresses don't have to be unique to just you at every moment, but you interent provider will provide you with one while it is in use and then rotate your address if you so choose.
To see more info on IP addresses see http://en.wikipedia.org/wiki/IP_address#IP_subnetworks