Dionysos is the son of Zeus, chief of the Olympians, and Semele, a woman of Thebes, according to the most used geneology. Dionysos is the god of wine and madness, vegetation, and the theatre, and was the focus of various mystery cults (which were not, perhaps, exactly cults, but let's not get into that).

Oh, Thebes, nurse of Semele, crown your hair with ivy! Grow green with bryony! Redden with berries! O city, with boughs of oak and fir, come dance the dance of god! Fringe your skins of dappled fawn with tufts of twisted wool! Handle with holy care the violent wand of god! And let the dance begin! He is Bromius who runs to the mountain! To the mountain! Where the throng of women waits, driven from shuttle and loom, possessed by Dionysos!
Euripidies, The Bacchae, trans. Richmond Lattimore
Dionysos is a strange god. Although he is best known as the god of wine, he is also a vegetation deity, a god of the liquid element, a death god, a god who comes into and changes, often irrevocably, the normal community life, and lastly the god of the theatre. His role as vegetation deity is obvious; he introduced the grapevine and taught the secrets of its cultivation and of fermenting wine. He is also associated with the fir tree and with ivy-- his symbol the thrysus, which his worshippers carry, is a branch or stalk of fennel tipped with ivy leaves or sometimes a pine cone. He is often paired with Demeter, goddess of grain.
Water is also a part of Dionysos's domain. The myth of Lycurgus, related in the Iliad, tells how a young Dionysos and his foster mothers were attacked and chased by the impious Lycurgus. Dionysos fled into the sea and was sheltered by Thetis. The god is also a sailor; there is a myth which tells how he was once kidnapped by pirates and taken aboard a boat. The god then turned the pirates into dolphins. The sea is a refuge for Dionysos. Water, according to W. F. Otto, is the element in which Dionysos feels at home, as like him it betrays a dual nature: being bright, joyous, and vital for life, while also having a side that is dark, mysterious and deadly.
Dionysos is the wine-god, and thus should be a pleasant fellow, a benefactor. But wine has both positive and negative aspects. It makes people drunk, causes them to behave in strange ways. The Greeks were well aware of the dual natures of wine, mirrored by the dual nature of its god. The story goes that Dionysos paid a visit to the house of a horticulturist, Ikarios. He left with this man a vine-plant, telling him that by following the instructions he would be able to extract from the plant an unusual drink. Ikarios planted the vine, harvested the grapes, fermented the liquid exactly as he had been told to. He then invited his neighbours over to taste the new wine. The fragrance of the drink amazed them, and before long they were singing its praises. Then suddenly the drinkers began to collapse, falling over in drunken stupor. Those left standing accused Ikarios of poisoning them, and they beat him to death and threw his mutilated body into a well. His daughter hanged herself. This, according to myth, was the first manifestation of Dionysos, benefactor of mankind, giver of good things.
Dionysos certainly roves more than the other gods; the traditional picture of him is not one of him sitting sedately on Olympus sipping nectar and listening to the Muses sing. Rather it is one of him roaming through the wilderness, thrysus in hand, followed by bands of ecstatic women, his Bacchants, and spreading the art of cultivation of vines and of wine-making. Other gods may leave Olympus, but it is not habitual with them as it is with Dionysos. Dionysos often seems to stand somewhere between male and female, between god and man, between death and life. He is a male god, but he is always surrounded by women, his chief worshippers. His worship involved transvestism and the blurring of sex roles. Men and women both dressed in long robes covered by fawnskins, and women, as bacchants, left their normal sphere of activity, the home, and danced madly on mountainsides. Dionysos even looks somewhat ambiguous sexually; Pentheus in the Bacchae comments on the god's effeminacy: his long curls, his pale complexion. Dionysos is also, unlike most of the other gods, the son of a mortal woman, Semele. This means that by birth he is a native son of two realms, the mortal and the divine. This theme also shows in Dionysos' marriage to a mortal woman, Ariadne.
According to the myth, as a young child, Dionysos was kidnapped by the Titans, who lured him with marvellous toys. While he is gazing at his own image in a mirror, the Titans slice his throat with a sacrificial knife. The child-Dionysos is then cut up into pieces and first boiled, then roasted. Zeus is attracted by the smell of cooking, and when he realises what is being cooked, he kills the Titans with a thunderbolt and resurrects Dionysos. According to some variants of the story, man then first appeared, born from the ashes of the burned Titans. So Dionysos is the god who dies and is reborn, and from his death... his sacrifice, for the titans follow correct sacrificial procedure when killing him, humanity comes into being.
But those at whose hands Persephone accepts atonement for her ancient grief, their souls in the ninth year she sends up again to the sun of this world � and for all time to come they are called of men holy heroes.
Pindar, fragment quoted by Plato.

In Orphic theology, Dionysos is the son of Persephone, Queen of the underworld, rather than Semele. Zeus remains his father; he is said to have impregnated his daughter Persephone in the form of a snake. The Orphics both identified the soul as separate from the body, and gave a reason for it being present in the body: it is being punished. The reason for the punishment is alluded to in the passage above. The ancient grief of Persephone, according to one arguement, is sorrow for the death of her son Dionysos at the hands of the Titans. Humans pay the punishment because they were formed from the Titan�s ashes and have a Titanic nature. By living an Orphic life and avoiding the bloodshedding which is the legacy of the Titans, humans may pay the penalty and achieve freedom.
Death forms a major part of the worship of Dionysos. In general, most of the Olympian gods seem to disapprove of murder and cannibalism. They reserve one of their harshest punishments in the underworld for Tantalus, who killed his son and served him to the gods at a banquet. Dionysos, by contrast, seems to revel in human sacrifice. There are a number of myths which involve women who he has driven mad as punishment who tear apart their children with their bare hands and later, occasionally, eat them. The best known example is that of Agave in the Bacchants. Agave, is running wild on the mountain with the rest of the women of Thebes, having been driven mad by the god, who is fighting to establish his worship in this city. Her son Pentheus, who opposes Dionysos, is lured by the god into going to spy on the women. Agave and her sisters rush upon Pentheus and tear him apart with their bare hands, scattering the pieces of his body over the mountainside. Dionysos's worship is thus established by the simple means of killing the opposition. But the story has deeper connotations as well. Pentheus in this case has been dressed up in the same bacchic costume of fawnskin and thrysus that the god himself wears. It is possible that he is serving as a stand-in for the god, dying the death of Dionysos at the hands of his mother rather than the Titans.
It has been suggested that every tragic hero who suffers and dies on stage at the Dionysia, the great dramatic festival at Athens, is in fact Dionysos himself, being killed. In conjunction with this, it has also been proposed that the sacrifice plot was the original plot of tragedy, and the festival of the Dionysia honoured Dionysos by re-enacting his death. It is interesting to note that a surprising number of children are killed in tragedy, as the child-god himself was killed. The most obvious parallel is with Atreus, who killed the children of Thyestes, cooked them, and then fed them to their father. This story is told in lost plays and is mentioned in Aeschylus's Agamemnon. Medea kills her children to revenge herself on Jason in the Medea of Euripides, Heracles kills his wife and children in a fit of madness in his play, also by Euripides, and Agamemnon sacrifices his daughter Iphigeneia to Artemis, as is told in Aeschylus's Agamemnon, among other places.
And there takes place every other year during the Agriona a flight and pursuit of these women by a priest of Dionysos holding a sword. And he is permitted to kill anyone he catches, and in our own time Zolius the priest did so.
Plutarch, Quaest. Graec.
According to legend, the daughters of Minyas refused to take a part in the dances in Dionysos' honour. In revenge, the god drove them mad. They developed a craving for human flesh, and drew lots to determine whose child they would devour. Leukippe drew the unlucky lot, and the Minyades tore her son Hippasus to pieces and ate him, raw. The women were later driven away, and the god Hermes transformed them into owls and bats.
The women were given the name Oleiai, or "Destructive Ones". The supposed descendants of this family retained the name, and Plutarch records that their banishment was re-enacted in a perverse, bloody ritual in which they were chased and occasionally, killed.
The duality of Dionysos is related to another of his attributes, which is that of loss of identity. The actors in the plays performed for Dionysos were masked; the mask symbolized the submersion of their identity into that of another. Wine also has the effect of submerging the normal personality of the person who drinks it. Dionysos made a habit of stealing the identities of his worshippers; the bacchants dancing on the mountainside have no separate personalities; they are mad, crazed, they have been taken over by the god; and they are all alike. Agave is certainly not herself when she tears her son to pieces, acting in unison with her sisters. In this respect they are behaving in the same way crowds often do, in which the individual is subliminated by the mob. Dionysos induces mass hysteria, he is the god of mob fury. This loss of individuality is demonstrated in the theatre not only by the masks which the actors wear, but also by the chorus. They dance and sing in unison, all chanting the same words. The members of the chorus have no identity, each is merely an insignificant part of the whole, with no separate will. All individuality and willpower must be given up to Dionysos, when the god choses to take it.