Gear shifting is the process of changing the gears in a vehicle to control its speed and power. In a manual transmission, the driver uses a clutch pedal and gear stick to select the appropriate gear based on the vehicle's speed and engine performance. This allows for efficient acceleration and deceleration. In automatic transmissions, the vehicle automatically shifts gears based on speed and engine load, making it easier for the driver. Both systems are designed to optimize engine performance and fuel efficiency, ensuring a smooth driving experience while adapting to different driving conditions.