Fabliau is the singular, and fabliaux the plural, and a fabliau is clearly and simply defined by Larry Benson in his introduction to the Riverside Chaucer:
"A fabliau is a brief comic tale in verse, usually scurrilous and often scatological or obscene. The style is simple, vigorous, and straightforward; the time is the present, and the settings real, familiar places; the characters are ordinary sorts… the plots are realistically motivated tricks and ruses. The fabliaux thus present a lively image of everyday life among the middle and lower classes. Yet that representation only seems real… the plots, convincing though they seem, frequently involve incredible degrees of gullibility in the victims and of ingenuity and sexual appetite in the trickster-heroes and -heroines.
(The Riverside Chaucer, p. 7.)
Fabliaux was primarily a French genre, and many examples still survive; there are very few fabliaux from Chaucer’s time written in English.
The Tales of the Miller, Reeve, Shipman, Summoner and Cook are all fabliaux, and many of the other tales, including the Merchant and the Wife of Bath, demonstrate the influence of the genre. Further reading in fabliaux might included Guèrin's Bèrenger of the Long Arse and the fabliaux of Marie de France, who seemed to parody her own writings in courtly love by writing fabliaux about the deceptive nature of women.
Characteristic of the fabliaux genre is its dangerousness and irreverence, often twinned in Chaucer with an almost-blasphemy:
"The cuckoldings, beatings, and elaborate practical jokes that are the main concern of the fabliaux are distributed in accord with a code of "fabliau justice," which does not always coincide with conventional morality: greed, hypocrisy, and pride are invariably punished, but so too are old age, mere slow-wittedness, and, most frequently, the presumption of a husband, especially an old one, who attempts to guard his wife's chastity… The fabliau, in short, is delightfully subversive - a light-hearted thumbing of the nose at the dictates of religion, the solid virtues of the citizenry, and the idealistic pretensions of the aristocracy and its courtly literature, which the fabliaux frequently parody, though just as frequently they parody lower-class attempts to adopt courtly behavior."
(The Riverside Chaucer, p. 8.)