What is an Irish Jig?:

A jig is a traditional Irish tune style (and the name of its accompanying dance) that dates back until at least the 1600s. It was originally a Continental European tune and dance, and the word "jig" comes from the French word "gigue," meaning "jump."

Meter:

A standard jig is characterized by 6/8 time, also called compound duple meter. Variations in the emphasis of notes (particularly when dancing is involved) have different names: a single jig, for example, tends to group the notes as two sets each of a quarter note and eighth note, whereas a double jig groups them as two sets of three eighth notes.

A slipjig is a variant in 9/8 time.

Non-Irish Jigs:

Jigs are also found in the traditional folk music of a few other cultures, though generally, they were brought to those cultures by Irish (or occasionally Scottish) immigrants. Quebecois music, for example, has a large number of jigs in its repertoire, as does Cape Breton traditional music, Acadian music, Ottawa Valley fiddling, and other genres of Canadian music. Cajun music of Southwest Louisiana also has a number of jigs in its tradition, as does Appalachian and Ozarkian old-time music.

Examples of Jigs:

Click to sample MP3s:
The Bothy Band - "The Ballintore Fancy" (Single Jig)
Le Ceoltóiri Cultúrlainne - "Rambling Pitchfork" (Double Jig)
Liz Knowles - "Hardiman the Fiddler" (Slipjig)