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Bevel Gear
Bevel gears are a common gear type in both the real world and the LEGO world. Bevel gears are flat on one side and beveled on the other, so that their bevels mesh with other bevel gears at an angle less than 180 degrees. In LEGO sets, they come in 90-degree sizes with 14 bevels, also known as “teeth.”
Crown Gears
True to their name, crown gears are crown-shaped; the bottom end is flat while pointed triangle shapes form a cup-like structure around the gear, which interlocks with other crown gears. These teeth are set in the gear's rim and rest perpendicular to its flat plane. In LEGO Technic sets, crown gears come in 24-toothed sizes.
Worm Gears
Though a somewhat obscure gear type, some LEGO Technic sets employ worm gears for pneumatic locomotion. Worm gears consist of two parts, a standard spur gear and a rod with a beveled surface that is something like a stubby screw. As the screw meshes with the teeth of the gear, it rotates. Worm gears operate on inclined planes and generally serve as driving gears.
Differential Gears
Differential gears employ two axes on a single line, using a small rotating gear to propel a larger gear into motion. Though these gears share a common driving force, the larger gear rotates faster than the smaller one. Rear axles on cars employ this gear type; similarly, LEGO Technics vehicle construction sets often employ differential gears in 28- or 24-toothed varieties, coming in both crown and spur shapes.
Rack Gears
A rack gear is a bar with beveled teeth that meshes with a straight beveled pinion rod—hence the term “rack and pinion.” When included with LEGO Technic construction sets, rack gears come in 1-by-4 sizes with 10 teeth.