Difference between single phase and 3-phase motors

The voltage, maybe.

Here's a good illustration of the 3 phases in a 3-phase motor.

http://upload.wikimedia.org/wikipedia/commons/3/3c/3phase-rmf-320x240-180fc.gif

Also see this thread: https://www.askmehelpdesk.com/electrical-lighting/3-phase-vs-single-phase-49262.html

The voltage, maybe.

It is the construction, not the voltage. 240 volt motors are fairly common in the USA in both single phase and 3 phase. Always and never are dangerous words. I will say other than an induction motor in 3 phase, 120 volt 3 phase, and 480 single phase are rare. If somebody else insists on unknown, I won't argue.

Only and only voltage cause.