Disney World has 4 major theme parks; Magic Kingdom, EPCOT, Hollywood Studios and Animal Kingdom. Within Disney World Resort there are also 2 water parks; Blizzard Beach and Typhoon Lagoon. There is also a 5 story arcade (Disney Quest), Golf courses and and the World Wide World of Sports.
Universal Orlando has 2 major theme parks; Universal Studios and Islands of Adventure. Also an entertaining area (CityWalk) and a Water Park (Wet 'n' Wild, 3 miles from the park). Included in Islands of Adventure park is the new Harry Potter World, probably the best ride in North America today, complete with a replica of the town, the Castle/School (Hogwarts), etc.
SeaWorld actually own 3 different theme parks in Orlando / Tampa: Sea World Orlando, the new Aquatica water park (across the street from Sea World) and Busch Gardens Tampa Bay. Aquatica is the newest water park in Orlando and have features and slides involving dolphins, etc like only Sea World can do. You can choose either a single park or combo tickets for more savings.
Besides the major theme parks, Orlando has 4 major Dinner Shows; Arabian Nights, Medieval Times, Pirates Show & Sleuths Mystery; they are a good way to start your vacation if your arrival time into Orlando doesn't allow for a whole day at the park. Kennedy Space Center is also a very popular attraction and Legoland Florida just opened their doors on October 2011. The Flexticket passes, will include most of non-Disney parks on one ticket.