Satyrs were woodland creatures in the form of men with some animal features. They were usually young and vigorous (today they would be called randy) and traditionally accompanied the Maenads in the revels of Dionysus. Hesiod says that they were descended from the five daughters of one Hecaterus, who had married an Argive princess, the daughter of king Phoroneus.
The Romans identified them with their own woodland spirits, the fauns, and it was thus the Romans who gave them the familiar goats' legs and horns with which they are depicted in post-Renaissance paintings. Popularly they were thought of (the image still lives) as perpetually pursuing maidens through the woods.
Their name has been associated with ther, 'wild animal' (Euripides certainly referred to them as theres) but it may actually be derived from some ancient Peloponnesian dialect word meaning 'the full ones', this referring to their 'abundant' or sexually excited condition. Eric Partridge suggests that possibly the 'sowing' root of sat- may lie behind the name.