Britannica Concise Encyclopedia
The word maenad comes from the Greek, meaning “mad” or “demented.” During the orgiastic rites of Dionysus, maenads roamed the mountains and forests performing frenzied, ecstatic dances, and were believed to be possessed by the god. While under his influence they were supposed to have unusual strength; it was said they could tear animals or people to pieces (the fate met by Orpheus). As bacchantes they were named for Bacchus, the Roman counterpart of Dionysus.
In Greek mythology, Maenads [MEE-nads] were female worshippers of Dionysus, the Greek god of mystery, wine and intoxication, and the Roman god Bacchus. The word literally translates as "raving ones". They were known as wild, insane women who could not be reasoned with. The mysteries of Dionysus inspired the women to ecstatic frenzy; they indulged in copious amounts of violence, bloodletting, sex and self-intoxication and mutilation. They were usually pictured as crowned with vine leaves, clothed in fawnskins and carrying the thyrsus, and dancing with the wild abandonment of complete union with primeval nature.
They also were characterised as entranced women, wandering through the forests and hills.¹ Also, they are described as mad women and nurses of Dionysus, wandering through the mountains. They went into the mountains at night and practised strange rites.² Confer also the descripton in Homer's Illiad, Book VI, beginning at line 130.
" ... he that on a time drave down over the sacred mount of Nysa the nursing mothers of mad Dionysus; and they all let fall to the ground their wands ..."
The Maenads were also known as Bassarids (or Bacchae or Bacchantes) in Roman mythology, after the penchant of the equivalent Roman god, Bacchus, to wear a fox-skin, a bassaris.
The behavior of Maenads in stories is intended to explain and display the intoxicating effects of alcohol. In some cases, the alcohol causes bizarre behavior in people and cannot be justified or explained by any other reason except that of the intoxication.
In Euripides' play, "The Bacchae", Theban Maenads murdered King Pentheus after he banned the worship of Dionysus because the Maenads continued worshipping Dionysus. Dionysus, Pentheus' cousin, himself lured Pentheus to the woods, where the Maenads tore him apart and his corpse was mutilated by his own mother, Agave. Culminating when she tears off his head, believing it to be that of a lion.
A group of Maenads also killed Orpheus.
In Greek Art the frolicking of Maenads and Dionysus is often a theme depicted on Greek kraters, that are used to mix water and wine. These scenes show the Maenads in their frenzy running in the forests often killing any animal they happen to come across.
See also Icarius, Butes, Dryas, and Minyades for other examples of Dionysus inflicting insanity upon women as a curse.
¹ David Wiles, Greek Theatre Performance: An Introduction. Cambridge University Press. 2000. 
² Katherine Lever, The Art of Greek Comedy, 1956.