From Wikipedia (and edited):

The plantain is generally used for cooking, in contrast to the soft, sweet banana (which is sometimes called the dessert banana).

The word "banana" is often used (some would say incorrectly, although there is no formal botanical distinction between bananas and plantains) to describe other plantain varieties, and names may reflect local uses or characteristics of varieties: cooking plantain, banana plantain, beer banana, bocadillo plantain (the little one), etc. All members of the genus Musa are indigenous to the tropical region of Southeast Asia, including the Malay Archipelago and northern Australia.

Plantains tend to be firmer and lower in sugar content than dessert bananas. Bananas are most often eaten raw, while plantains usually require cooking or other processing, and are used either when green or under-ripe (and therefore starchy) or overripe (and therefore sweet).
Source(s):
http://en.wikipedia.org/wiki/Plantain

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they are a member of the banana family, but are more starchy than those that are eaten as a soft fruit. they're usually cooked, fried, mashed and used as ingredients in other recipes.
they still have a certain sweetness, but i'd more liken them to something similar to sweet potato than a common banana.
Source(s):
if you google "plantain" you'll find many links, but here's the wiki:
http://en.wikipedia.org/wiki/Plantain

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Very similar to the Banana but the texture is rougher and not very sweet.

Lots of carribean meals include plantains such as Tostones.

They are delicious when they are freshly fried next to steak, blackbeans and rice.
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They are like larger bananas with tougher skin and taste similar. They are mainly grown in Central America and are used in a popular dish there called Tostones (fried plantains).
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