The turns rating is a (very) rough method of indicating power and current draw. Basically, it's the number of times a wire is passed around the magnetic core of the motor, and the lower the number (8 turns is pretty much as low as they get currently) the more current it draws, the higher the speed but conversely the lower the running time and the poorer the torque. A standard motor (at the reverse end of the scale) is a 27 turn - lower speed, low current draw, long run time, high torque.

The second number in a motor rating (e.g. 27x1, 13x2, 10x3) is the number of wires that are used in the wrap.  So a 27x1 has one wire wrapped 27 times around the core, a 10x3 has 3 wires wrapped 10 times.  The lower the number of turns (e.g. 10) the lower the torque generated so many manufacturers add more wire loops (e.g. x3 x4 etc) to increase the magnetism and therefore the torque.

Compatibility car wise, we don't recommend a motor lower than 17 turns in a shaft driven car (they tend to overheat the gears) but many get away with it.  Belt driven cars are able to take a higher power motor with less risk of damage to the gearing.

When applied to speed controllers, please bear in mind that a worn standard motor (a notional 27 turn) or a standard motor used in extreme conditions (for example on an off road car negotiating a rough ground or long grass course) will draw more current.  It is therefore wise to purchase a higher capacity Electronic Speed Controller (for example buy an ECO20 in place of an ECO27 if running an off-roader) so that the electronics within are not running at their limit.  This will reduce the likelihood of cut-out or overheating.