The percussion section includes any number of instruments from timpani to tubular bells and from castanettes to congas. The variety of instruments is enormous, however they do fall into two main groups: tuned percussion and untuned percussion.
Percussionists play both tuned and untuned percussion instruments, but some players will specialise in certain types of percussion and may become particularly expert on certain instruments.
The timpani are something of a special case. Since they are the percussion instrument most frequently used in orchestral music, they are usually played by specialist players who may or may not play other percussion instruments.
There can be any number of percussion instruments in a composition and the variety of instruments used has increased enormously since the beginning of the 20th century.