Stock motors have plain bushings, fixed timing, and 27 turns of machine-wound wire. A stock motor can refer to the motor that came with the car kit.
Modfied motors have ball bearings, custom winds, and adjustable timing.
Before purchasing any modified motor, be sure your speed control can handle the extra current that modified motors demand. Never use a modified motor with a mechanical speed control and always heed the manufacturer's recommendations for the number of turns your ESC can handle. Double, Triple, Quad, etc., refers to the number of strands in the wire that is used. Fewer winds usually equals greater rpm's and shorter run time. More strands usually means smoother acceleration.