There are three nighttime shows that include fireworks at WDW:
1. Wishes at Magic Kingdom
2. IllumiNations: Reflections of Earth at Epcot
3. Fantasmic at Disney Studios
As described in the last post, check the park schedules to find out what the showtimes are. They vary according to the season, and according to park hours.
I believe Wishes is the only one of the three that's predominantly fireworks. Haven't seen it myself yet, so I don't know what else they have in the way of effects.
IllumiNations is a light/laser/water/fire effects show on the World Showcase lagoon, that has some fireworks at the beginning and end.
Fantasmic is more of a 'show'. You're seated in an amphitheatre, and watch Mickey do battle with various villains. The show also includes light effects, water screens, etc. There's a boat full of characters at the end, and I believe the fireworks are also at the end.
All three are well worth seeing.