What is the difference between dedicated servers and non dedicated ones?

[ps3-sales!]

I've been hearing a lot about the decision to drop dedicated servers in the pc version of MW2. This sparked the question in my mind, what exactly is a dedicated server? Because, well, I really don't know.

Could you guys give me some examples or something of games that have dedicated servers vs games that dont have dedicated ones? Preferably console games as I am not a pc gamer.

Thanks a lot!