[morzh]

The difference is, the async has the frequency of rotor rotation not equal to frequency of stator's field rotaition, and is used for smaller power apps whereas synch motor has the frequencies equal. When you see a passive rotor (a squirrel cage or such) - this is an asynch.
It is the most popular motor today.

PS. I don't pick on anyone, especially Brenda Ann. She's a wonderful girl.