Need a power yet compact worm gear motor? Why is worm gear so special? Check this wiki out: Worm Drive.

" Unlike with ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible when using large reduction ratios, due to the greater friction involved between the worm and worm-wheel, when usually a single start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input."

In other words, it is difficult to push your load (example, your robot) if the motor is not power or rotate.

This is 12V rated worm gear DC brush motor with gear ratio

Features and Spec:

• Uses a metal gear box
• Durability and a high torque output
• Output shaft is self-locking due to worm gear
• Rated voltage: 12V DC
• Rated Current: 0.6A
• Rated output speed: 10- 375 RPM +- 10 %
• Rated output torque: 1.2 - 22.5 kg.cm
• Max torque: 25kg.cm
• No load current: 60mA
• Stalled current: 1.3A
• Output shaft diameter: 6mm with 4.5mm slot