Dedicated servers

What exactly are they? I hear how they are good, and allow a community of online players to become more familiar with one another, and so on. But I'm still not entirely sure what they are.

Thanks in advance for replies!

I think it also allows whoever owns it to set the rules and stuff like that. They determine the map, what mode, and they can throw in whatever mods they want. Its pretty cool, whenever I get tired of playing the normal modes and maps that come with the game I can go into some other server for a completely new experience.