Electric Motors convert electrical power to rotary power, which can then be shaped into accomplishing whichever task needed. The standard Tetrix parts kit comes with its own 12v DC motor, but others can be purchased from different sites. Seemingly the most popular model so far are the AndyMark NeveRest 40 Motors, but stocks run out frequently.
Attaching motors to your chassis requires a motor clamp, and might require the use of a shim to ensure a snug fit. After you have found an efficient place to mount your motor, decide on how to power the tool you want to move. This can be done by using gears, chain and sprockets, or worm gears.