Timpani are sometimes called 'kettle drums' due to the bodies being made of shiny copper which makes them look a little like large cooking pots. Each drum has a covering, or head, that is stretched very tight.
By either tightening or loosening the head, the drum can be tuned higher or lower, respectively, to play in the same key as the other instruments of the orchestra. This tuning used to be done quite tediously by turning several screws, one at a time, until the desired tuning was achieved. Modern timpani are fitted with a foot pedal mechanism that allows uniform, quick tightening or loosening of all screws simultaneously.
Timpani originated in India and came to Europe around 1300. Crusaders came across them being played by Turks in the Holy Land where they were played on horseback to encourage the soldiers. European kings and nobles would thereafter have them at court where they played fanfares along with brass players, providing the tonic, bass notes of each chord.
They became part of opera and symphony orchestras during the classical period. Some say that Haydn wrote the first solo part for timpani in his Symphony Number 103. The effect was so startling that the work was nicknamed the "Drum-roll" Symphony.
Classically, there were two drums played together, tuned to the first and fifth of the key signature. When the key changed, the player would rest and/or tune the drums to the next pitch by putting his/her ear to the head and change the pitch while playing very softly. Early calfskin drum heads would stretch and change pitch while playing, so the player was constantly listening between passages and making the appropriate changes to stay in tune. Large orchestras can use up to 5 timpani of varying size, arranged in a curve around the player.
Timpani are played by making quick, staccato strokes with a felted mallet, each strike landing between the center and the edge of the drum, then the head is usually damped with the butt or side of the palm to make the sound stop.
The most characteristic sound made by timpani is the drum roll where each mallet is used evenly in a rapid back and forth pattern...this can be a very exciting effect if it gets louder as it is done. It's too bad that the percussion instruments are behind the other instruments...they can be very entertaining to watch.