CAA: Christian Anime Alliance

This is one of those questions that isn't really important but bugs me in the back of my mind kind of way.

What exactly does an ISP do? I know that I have to pay them to use the internet but what exactly am I paying them for?

http://computer.howstuffworks.com/internet-infrastructure.htm

Some simple, yet indepth, reading on the matter.

(It would take considerable time to properly describe it myself.)