Why do cats shake their tail really swiftly?

My cat is just over a year old and when we wake up surrounded by the morning to feed her, she shakes her tail really fast.. Kind of like it's twitching.She does this during the time too if we're in one room and she's in another and she see us walking into the room. Does this mean anything, because it seems to me like she is wanting attention. I know it may give the impression of being crazy, but I thought it was something to ask.
My cat shakes its tail fast when it is angry.In other cases when it wishes your attention
This shows excitement and anticipation of something pleasant and something she is looking forward to. My kittens do this flipping back/forth when I go to pat them. They also anticipate the pets and get excited. Cats are very affectionate creatures and love virtuous relations with their owners. Quality, loving companionship is very important to cats.
Answers:    She's really excited. Some do it when they're pissed too but if there's some food to be eaten and attention to be have she's probably excited.
my cat flips her tail put money on and forth when she is trying to decide if she wants to progress outside or not. i call it her indecisive tail wag.
I've see cats shake/ vibrate their tails when they see food or when they see something they want to play with. They also do it when they're concentrating on catching something.
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