It looks like a big banana, but it's much starchier and less sweet. It's used a lot in West Indian/Caribbean cooking, usually sliced and fried. It's really nice though maybe an acquired taste if you've never had it before.
I think it's a fruit, as it's more or less like a banana... though I've been told that technically a banana is a herb, but whether or not that's an urban myth or not, I don't know.
I found this info on the internet (veggiesmatter.co.uk) if it's of any use to anyone:
This popular banana in Latin American, Caribbean, and Asian countries is often referred to as a cooking banana. Plantains resemble bananas but they are longer in length, thicker skinned, and starchier in flavor. In most countries, plantains are used more like a vegetable than a fruit. They are not suitable for eating raw unless very ripe, when they turn completely black. One half of a large plantain is low in sodium, and high in vitamin A. This versatile fruit has three unique stages when they can be eaten.
Green plantains taste more like a potato with a starchy texture. At this stage, the interior is yellowish or slightly pink. The fruit is firm and is often used as side dishes.
Yellow plantains are the middle stage of the fruit. These plantains can have some brownish-black spots. Their role now is both vegetable and fruit and is used in dishes that request for a slightly sweet taste and firm texture.
Black plantains are typically found in sweeter recipes. These plantains are all black or spotty black and are soft. Black plantains can be eaten out of hand.