Gears carry the motor's turn to the axles. A higher ratio (such as 5:1) accelerates harder; a lower one (such as 3.5:1) reaches a higher top speed. Which gear sets fit depends on the chassis.
For how each kind of part is used and tuned, see Tamiya Hong Kong's Mini 4WD setting guide (in Chinese).
22 parts
Product pictures are thumbnails of TAMIYA's official photos and remain TAMIYA's copyright. This is an unofficial fan site with no affiliation to TAMIYA.