Although cats are often mysterious creatures, you don't have to be in the dark regarding their enigmatic ways. Cats, like dogs, express emotion by moving their tails around. Feline tail wagging, however, isn't a surefire indication of a happy and content cutie, so make no assumptions.
Irritated
If a cat's tail is quivering ever so slightly, it might mean that he's irritable about something. If that annoyed feeling escalates, the quivering might turn into tail beating, in swift and harsh motions. Cats frequently do this while they're sitting. This type of tail wagging usually points to a cat who just isn't a happy camper at the moment. If you're stroking your pet's back, this wagging probably means something like, "I appreciated it initially, but I'm 100 percent over it now. Stop, please."
Dispute
Cats frequently wag their tails when involved in disputes. If a cat wags his tail, it could mean that he's about to get aggressive with another cat. It also could mean that he feels intimidated by another individual's actions and is about to go into protective mode. Always be cautious when in the presence of cats in these moods. Tail wagging in these situations is usually quick, and is often accompanied by other key "angry" body language signs -- pupil size changes, pressed back ears and uncontrolled hair bristling, for example.
Enthusiasm
If a cat's tail is pointing straight up and is shaking just a wee bit, it could mean that he feels simultaneously enthusiastic and overwhelmed. Maybe you just brought him his favorite toy and he knows that it's playtime. Maybe he sees the yummy treats in your hand. This kind of wagging sometimes can even be a sign of a cat who is on the verge of urine marking.
Feeling OK
Light and subtle tail wagging sometimes is a sign of a cat who feels, simply put, "OK." He may be feeling calm and nothing too interesting is going on. Although the little guy might not be particularly overjoyed at the moment, he's in a serene and placid mood with no worries on the horizon.
Uncertainty
Extremely repetitive yet moderate tail wagging sometimes means that a cat isn't totally sure what to make of his present environment and is trying to make sense of what is going on. This cat probably isn't upset, but at the same time probably isn't in his comfort zone either. He's probably mulling over his next action.