Questions on roms,themes an o/c's

[ChevyNexus]

Ok so I hope that I posted this in the right section if not I'm sorry.

So I rooted my droid awhile ago an then got super busy with work an side jobs so I never got a chance to do roms or theme my droid. Well I injuried my shoulder so now I have some free time. An I figured i'd do it now but I'm really confused on how an what to do.

So I was hopen I could get some answers. =)

1) what is a rom? How do you install them? What do they do to your phone? What's the benifits of running roms?

2) what are themes? How do you install them?

3) when I rooted my droid to froyo 2.2 it had a mydroid world kernal how can I change that to a chevy o/c kernal?

So hopefullly I can get some answers to these an maybe a guide on how to install roms an themes.