A shiny pokemon is a differently colored pokemon, it's basicly the pokemon equalivent to albino animals, you get them randomly and consiter your self lucky getting a shiny, there is a 8 in 65536 chance of finding one in the wild without chaining. :)
I'm pretty sure a good half of them are some sort of green. Which looks really, really bad on most Pokémon. A few of them look like they barely change at all; Look at Shiny Leafeon. It's just a little bit more vivid green. Hard to tell.
There are some that are just so, so awesome. xD LIKE RAYQUAZA. Omg. Go see.