There have been lots of threads about it; try a search.
The party is in the lower level of the Tomorrowland Terrace Noodle Station (or whatever they're calling it this week). The QS restaurant is closed while the party is going on. The party starts an hour before the fireworks. Private tables, lots of lovely little desserts and various (non-alcoholic) beverages (coffee, iced tea, milk, lemonade, but no soda pop). You cannot really watch the parade from the Terrace but you get a very nice view of the fireworks; many people gravitate up towards the railing but you can also see just fine while sitting at your table. No one else can come into the area during the fireworks. The buffet is open from 1 hour before the fireworkst to 10-15 minutes after they are over.
It is not a credit on any of the Dining plans. Like a few other special experiences/shows, your credit card is charged when you make the reservation.
Just like everything else at Disney, it's a great experience for some people while others would hate it.