Universal Orlando Resort is made up of three parks: Universal Studios Florida, Islands of Adventure and the recently opened water theme park Volcano Bay. Walt Disney World consists of 4 theme parks: Magic Kingdom, Epcot, Animal Kingdom and Hollywood Studios. It also has two water parks.
This means if you want to go from Epcot to Islands of Adventure you are going to have to have a different ticket for each. The same also goes for Sea World, Legoland and Busch Gardens. All of these parks will require their own tickets to enter.
If on your vacation you want to go to multiple parks, it may be worth looking attraction ticket websites for good multi park options. Attraction Tickets Direct have a number of combination packages that cover all the major theme parks in the Orlando area.