Source: http://en.wikipedia.org/wiki/Timpani.htm
The timpani is basically a set of tunable drums, similar in structure to the kettle drum. The pipe-shaped body is usually made of copper, aluminum or fiberglass. As with the traditional drum, a vellum is stretched over the body pipe. The vell'um is struck with a mallet. The larger the diameter of the pipe, the lower the frequency.
Timpani's are usually tuned to specific desired pitches, depending on the tuning system to be used. This is quite a job. Modern timpani's have pedals that use various methods to speed up the tuning exercise.
Source: http://en.wikipedia.org/wiki/Timpani.htm
http://www.youtube.com/watch?v=bH1q3orIYUU