To amend JK's post: there are many different types of turns in use. What notes they are made of may depend on the period the piece is from or on the composer. Also, these ornaments are sometimes executed starting BEFORE the beat of the principal note (e.g. in Chopin), sometimes AT the beat (e.g. in Bach). There are exceptions everywhere, and often the interpretation of ornaments is a matter of heated debate among scholars. Ornaments are usually used to emphasize a certain note over, say, the bass line. In Baroque music, this has to do with the limited dynamic capabilities of the harpsicord, which made it necessary to draw attention to certain notes through the use of ornamants. When the same pieces are played on a piano, one can often dispense with the ornaments, because one can properly shape phrases through dynamics (e.g. by playing the melody notes louder than the bass line).