Disc brakes rotors come in different diameters and in different mounting designs. The most common mounting design is called six bolt, which is where the rotor has six bolt holes in the middle is held on to the hub by six bolts (funny that). There is another mounting design called Centrelock which is where the rotor is splined in the middle and is held on to the hub via a lock ring. There are converter products available should you wish to run six bolt rotors on Centrelock hubs or vice versa. The diameter size of a rotor dictates how much braking power is ultimately available. Larger rotors offer stronger braking. If you're changing rotor size from an exisitng setup then you will also have to get a new mount bracket that matches the new rotor size. Rotors, mounts, brackets, mineral oil, bleed kits, bar-shifter mounts and various adapters.