Playsets are individual six hour "campaigns" based on individual Disney movies. Only characters from that movie will be playable in the playset.
So far, only four playsets have been officially revealed, three of which are in the Starter Pack (Pirates of the Carribean, Monsters University and The Incredibles) and one which will be available at launch as an additional purchase (Cars). It has been suggested that future playsets will include The Lone Ranger (with the Lone Ranger and Tonto characters), Toy Story in Space (with Jessie and Buzz Lightyear) and Phineas and Ferb (with Phineas and Ferb.) Each playset has its own unique gameplay features, so no two will be alike.
Players will find red capsules while exploring/playing through each Playset. These are customization items that they can use to customize characters and other objects in that particular playset. For example, players will be able to customize Jack Sparrow's ship in the Pirates playset with these red capsules.