The trombone is probably the most easily recognized brass instrument, due to its unique slide. It's also a particularly difficult instrument to play, because of the variety of different lip positions required to elicit the full range of notes it can produce. (This is a characteristic common to all brass instruments, but the trombone takes it to extremes.)
Trombones with valves do exist, but are less common than the more familiar slide-only version.
Here's a collection of pointers to various places in the Web that discuss the trombone: