Editing Maps?

I've seen maps edited, and people talk about editing them, but exactly how can you edit them? Cause when you extract the .rez, they come out as DAT files. And I have no idea on where to go from there. Any ideas?

i also would like to know this as i am tryin to remove the fog from snowvally via modding but i have no idea what files snowvalley uses

You have to find each and every item in the rez file

Textures.Rez