What is the difference between a mountain bike and a ten speed?
A cruiser bike is a bike that gives an upright riding postion,
is usually blinged out, and is ridden rather sedately.
MTBs are available in several different versions(XC, AM, DH),
but basically they're about providing a compromise between
ruggedness and rideability.
Mountain bikes have much knobblier tires and usually have
suspension, which is rarely available on cruisers. Mountain bikes
are also used competitively.