Fantasmic is a show at DHS. You book the Fantasmic Dining Package at either Mama Melrose, Hollywood and Vine, or Brown Derby. There are a limited number of packages and the dinner times are usually quite early. There's no additional charge, you just get a pass to the Fantasmic show with "VIP seating." You can still see the show, just get there earlier. And note that Fantasmic does not show every day.
Illuminations is at Epcot. I don't know of any dining packages associated with it, unless you do one of the Illumination cruises, which I don't know much about. You just stake out your spot and enjoy the view.
