Well, technically there are five expansions, but logically there are only three. Shrouded Isles, Trials of Atlantis, and Catacombs (and the upcoming Darkness Rising) are all the official expansions you need to pay for. New Frontiers is more like just an update for the RvR frontiers, and Foundations is if you want a house or want to visit a friend's (or guild's) house. Both of those are free and can be downloaded from the company itself.