Application Server / Application Pool

[hso]

I'm in the process of learning the Application Server role on Server 2008 but the definition I'm coming across do not have me in a state of understanding.

The following is the definition that I have most commonly come across:

"Application pools consist of one or more URLs that are served by a worker process or a set of worker processes. They use process boundaries to separate applications."

Ok, so its one or more URLs, but "served by a worker process or a set of worker processes"? What do they mean by worker processes? Can anyone give me a better explanation of Application Pools and an example please?

Thanks.