Bungie has addressed this question in Game Informer's month of Destiny event back in December. Bungie described that the different activities will have different difficulties. For example, the story is said to be the easiest, designed to be something to slip into and enjoy, while strikes bump up on the difficulty if your looking for something quick and fun. Raids have been said to be lengthier than strikes, and also more difficult. PvP uses flexible matchmaking, so you'll be fighting people around your rank/level.