A cat that wags its tail is not happy. This motion usually indicates a desire to be left alone. If the wagging escalates to thumping the ground, the cat may be warning of an impending attack.
Instead, to communicate a calm attitude, a cat's tail may curve down and up to form a gentle U shape. When a cat is particularly happy or wants to encourage interaction, it may hold its tail high and straight and give a slight flick of the tip. Thus, in general, the higher a cat holds its tail, the more content and approachable it feels.