Everything inside it goes with it. Deleting a class removes the class, all of its subjects, every card in those subjects, and any deadlines linked to them. Nothing is left orphaned.
Reps asks first, and the confirmation names the damage rather than asking a vague question. It reads along the lines of "Delete class 'Biology 111', all its subjects and cards and 3 linked deadlines?" so you can see the scale of what you are about to lose before you agree to it.
Deleting a single subject behaves the same way: the subject and all of its cards go together, with the same kind of confirmation.
A confirmed deletion takes effect immediately and propagates to your other devices the next time they sync, so the export is what gives you a way back.
Export the subject as a .reps file first. That file is a complete copy of the cards, and importing it later brings them back. It is the only backup Reps offers and it takes a few seconds.
This matters most at the end of a term, when the temptation is to clear out finished classes. A .reps file costs nothing to keep, and storage is not the reason people regret deleting a class.
More answers on the help page, or write to [email protected].