I'll try to explain them as simple as possible:
Griefing = when someone comes to you and starts doing everything he can to prevent you from enjoying the game (e.g. keeps spamming you, stalking you, plays all kind of stuff through the CB radio just to annoy you)
Cheating = using hacks like speedhack, teleporting around etc. (money cheats are still allowed, because MP doesn't have economy system yet and it doesn't use anything related to your in-game money)