Son of Zeus and Semele and god of wine, vegetation, fertility and often celebrated at theaters. He surrounded himself with maenads (orgiastic women) and satyrs, and held constant festivities in the forests. Anyone who angered him was struck with madness.
In art, Dionysus was depicted wearing a wreath of vineleaves, and holding the so called thyrsos rod in his hand. Dionysus was also connected with the seasons and the death- and resurrection beliefs of ancient times. His worshippers tried to reach a point of extasis (to stand out of ones body) and wine was an important factor in his rituals and the achievement of exctasy. He was often depicted on the Greeks sarcophaguses, and he was connected to the belief in immortality.
Dionysus was a foreign god from the East, and came to Greece through Thrace. In mythology, his birth is quite remarkable, since Semele died before she gave birth to him. Zeus took the embryo out of its dying mothers womb, and put it in his thigh. After Dionysos was born out of Zeus leg, Hermes took the baby to nymphs on the mountain Nysa that brought him up. This scene can be seen in the famous statue of Hermes and the baby Dionysus in Olympia.
Dionysus was often celebrated at the harvests of the grapes, and each village would have annual Dionysus festivities. He was strongly connected to the island Naxos, since he was said to have come across the by Theseus abandoned Ariadne and to then have married her.

Dionysos was also the god of drama, especially tragedy, since this theatre was said to have been invented by the satyrs. They would sing and play roles, and the very word tragedy means "goat song". Dionysos' drunken party that followed him around was called Komos, and from that we have the word comedy, which means "song by drunken party". The Great Dionysia were annual festivals in Athens where dramatists competed with their plays. The god was also connected to the orphicism, again a mystery cult having to do with immortality and resurrection. See Dionysus Zagreus for more on this.
The Romans called Dionysus Liber, but the Greek name Bacchus was more often used by them.
Dionysos also had many epithets:
Acratophorus, Acroreites, Aesymnetes, Agrionius, Amphietes, Antheus, Aroeus, Bassareus, Brisaeus, Calydonius, Cissus, Colonatas, Cresius, Eleuthereus, Hygiatis, Iatros, Lampter, Laphystius, Larymna, Limnaea, Lysius, Meilichius, Melanaegis, Melpomenus, Mesaetus, Methymnaeus, Mystos, Nyctelius, Nysaeus, Omaclius, Orthos, Psilas, Saotes.