The trap or basket collects debris from the pool water, as it passes through the system. This prevents clogs in the pump impeller.
The impeller is housed in the vacuum chamber. Attached to the motor shaft, it spins inside this chamber, moving the water through the pump. As the impeller turns in its chamber (or diffuser), this creates the water velocity output for the pump. Open-face impellers are bronze, while closed-faced impellers are plastic.
When a motor is replaced the parts of diffuser and impeller may be salvaged. On larger cast iron or bronze pumps this would be recommended in an effort to reduce costs.