The explorer class of the ROV competition is designed for higher level participants -- usually universities, and occasionally advanced high schools. Tasks are similar to the ranger class, but increased in difficulty in terms of requirements and size, and decrease in aid. Explorer vehicles require you to have more than one camera, as well as many more attachments in order to perform the high level tasks that the ranger and explorer classes requires.
Another factor that differentiates ranger from explorer is that explorer vehicles are allowed 48 volts, as opposed to ranger's 12. Explorer class participants also do not compete in a regional competition, and only compete at the international level.
The Sea Sweepers chose to move up to the explorer class after success in 2016 in the ranger class. They will compete with BRIAN, their only explorer class vehicle, in Long Beach for the 2017 international competition.