The title percussion instrument implies a musical instrument that is played by being hit. This is true, but it is
basically any instrument that relies on mechanical vibrations of the instrument or parts of the instrument, except for stringed instrument. They are separated into instruments with a
definite
pitch, and indefinite pitch. Instruments with a definite pitch include the xylophone and the timpani drums, those with an
indefinite pitch include the snare drum and some of the smaller "auxiliary percussion" instruments, including the tambourine and the
xylophone.
Percussion instruments are basically all about vibration and resonance. In a drum set for example, there is a cylinder which is the body of the drum with a piece of taut material at the top known as the drum head. There may also be another head at the bottom of the drum. When you strike the drum, the head deforms, but the tension causes it to snap back into place, vibrating the air inside the drum. The body of the drum also begins to resonate, as well as the head on the bottom of the drum if it is there, causing a loud sound to come from the drum.
The reason that you cant find a pitch for some instruments is that there is too much going on! Maracas for example are hollow instruments filled with beads that collide and resonate with each other and the body of the instrument. Snare Drums don't have a definite pitch either. These drums consist of a body of steel or wood, with a thick head at the top and a light plastic head at the bottom. Also on the bottom are the snares, taut chains usually made of metal, but sometimes plastic, that's also vibrate when the drum is struck, producing its distinctive sound.
For percussion instruments with a definite pitch, there are a number of slightly complex looking formulas that will give you the frequency of the instrument: