Dedicated Servers and What I Don't Know, HELP!

I’ve never done anything involving a dedicated server and don’t know a thing about it. What I’d like to know is are they like your own password protected multiplayer game that is open to you and your select friends at all times? Do they interfere with your single player/co-op games at all? Can I run a dedicated server and still play single player is what I mean. I’m really wanting to play with a friend or two, but don’t want to disrupt our game by still playing in co-op without them. Any help with this would be greatly appreciated.

As I’ve said, I know nothing about them and am looking into maybe getting one for us if I can still mess about in single without them.

Thank you for any response that is helpful.