A tambourine is considered a percussion instrument because the definition of this kind of instrument includes any that “makes a sound when it is hit, shaken, or scraped.” A tambourine can make sounds when it is hit and/or shaken.
For example, you can hold the tambourine with one hand and hit it with your other hand. Or, you can hit the tambourine on your hip. It can also be mounted on a drum set and hit with a drum stick. Perhaps one of the best known tambourine players is Stevie Nicks of Fleetwood Mac fame.