Garrys Mod, what is required?

[BeaverMon]

I'm interested in this "Garrys Mod" thing, but I want to know a few things;

1. Do you have to "play" online, or can you "play" offline?
2. Do you need anything else then HL2 to run Garrys Mod?
3. Is Garrys Mod easy to use for a n00b?
4. Can you play in the default HL2 Levels threw Garrys Mod?

[Pesmerga]

1. You can start your own game by yourself, but there's no fun in that
2. No.
3. Yes.
4. I think, but it's buggy.

[BeaverMon]

Thanks, man. :)