In a **drum brake**, the main moving part is **the drum itself**. This braking system works when the **brake shoes** or **brake pads** are pressed against the drum, which is connected to the wheel. When the brake is applied, the drum rotates and the shoes rub against it, generating friction that reduces the vehicle's speed or stops it.

- **The drum** is the part that moves along with the wheel, and it is essential for the functioning of the drum brake.
- The **brake pads** are not the moving part in this system; their function is to press against the drum.
- The **brake fluid** is not a moving part; its function is to transmit the force from the brake pedal to the shoes or pads.

Therefore, in a drum brake, the drum is the moving part that directly interacts with the shoes to stop the vehicle.