The EWP has a series of barges that cruise out on the lake and have colored lights that are arranged to form the shapes of different things (dragons/dolphins for example) and they are choreographed so they look like the animals are moving. The show is set to music. You can see it from a number of places: Ft. Wilderness, Poly, Contemporary, Grand Floridian...maybe others. We watched from the POly beach. Do a search and you will find the approximate times that it will be at each location.
Some of the postings I've seen indicate that it lasts about 20 minutes, but I think it was more like 7-10 minutes.
It was nice to see, but if you have to go out of your way to see it, keep in mind the time you'll spend to get to and from a 7 minute show (and it isn't that spectacular). But, knowing that, you can plan to incorporate it into other plans (for instance if you planned to resort hop, just to see what the different places look like, plan to be at one of the resorts when the EWP passes by). Or if you wanted to ride the monorail, do that in the evening and stop off to watch the EWP.