Cats wagging their tails..

People say cats wag their tails when they are aggitated is I am correct, but does it always necessarily mean they are mad? My one cay who is obsessed with me wags his tail a lot, even when he is purring. He'll lay on my bed next to me and smack my arm or body with it. I notice he does do it when he is pissed also though, but can they still do it when they are happy?