The pinion gear in an RC car is the gear that is attached to your motor, and the spur gear is the gear that the pinion drives. The spur is sometimes integrated with a slipper clutch or with a differential.
The ratio of the spur to the pinion is typically known as the gear ratio (spur teeth / pinion teeth). You can use the ratio to figure out if your car is geared higher or lower than other cars. For some cars, you may need to look beyond the pinion and spur to the final drive ratio (FDR). For cars with foam tires, you'll need to look at the rollout to get the complete picture.