The result of a clever idea of the 19th century: put a clarinet mouthpiece on a brass tube, combining the flexibility of a woodwind with the power of a brass instrument. Like most wind instruments that come in multiple sizes but share a common fingering system, it is a transposing instrument, so if you play a C scale (on paper) on the various saxes it will sound in Eb (on the alto and baritone), in Bb (on the tenor and soprano) --this often confuses beginning players but is a sensible idea for making the instruments easier to learn. Sounds great in the hands of a master, as VV notes, but of all the wind instruments the tone probably sounds worst when badly played, maybe because it can be so loud and piercing.
