ISP question

[muskrat]

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?

[Authority3000]

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.)