Gears are typically classified based on the orientation of their shafts and the shape of their teeth, including spur gears, helical gears, bevel gears, and worm gears. Spur gears, with straight teeth mounted on parallel shafts, are the most common and efficient but can be noisy. Helical gears feature angled teeth, which allow for smoother and quieter operation, making them suitable for high-speed applications. Bevel gears are used to change the direction of rotation between intersecting shafts, while worm gears are essential for applications requiring high reduction ratios and self-locking capabilities.