what the hell is a GCF/NCF and how do i use it

having trouble with using these

ok so i checked out threads with GCF's and this is what i've done. I downloaded a CS:S client, by pressing save. I put that into steam apps, and then i downloaded the undead thing, by pressing save. I cant open undead for some reason.

Edit:Installed WinRar, everythings fine now, i can't belive this works! Where should i go to find cracked servers for games?