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Okay, so hopefully you're not trolling here, but from your question you have no clue as to how brakes work and have heard the term "rotor" and that it is somehow related to brakes. So to educate you on the subject, here is what you need to know about brakes when you take the car in for replacement of them.
There are basically six components to the system:
Brake Pads - consumable - two (2) for each brake
Brake Rotor - consumable - one (1) for each brake
Brake Fluid (hydraulic oil) - consumable
Brake Caliper - repairable if required
Brake Master Cylinder - repairable if required
Brake lines - repairable if required
Brakes use hydraulic force to apply a friction material (the Brake Pads) against a cast iron disk (the Brake Rotor) that is attached to the axle. The hydraulic force is applied to the pads by the Caliper, which clamps the pads against the rotor (think of it as grasping a dinner plate if the dinner plate is in the vertical position). The hydraulic force is applied to the Caliper by your foot pressure on the brake pedal, which pushes a piston in the Master Cylinder. Pads are the friction material and are designed to be consumed and are generally what is replaced during a brake job. Rotors are also designed to be consumable, but generally not at the same rate as the pads. Hydraulic brake fluid is meant to be reguarlary replaced (every 24 months) as it collects water and dirt and becomes less effective in transfering the hydraulic force to the caliper.
For BMWs my advice is to replace the brake pads and rotor as a set because generally the rotor doesn't last all the way through the life of the second set of brake pads and may get too thin and warp over time.
Any independent BMW repair shop can provide brake service at a less price than the dealer.
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