Freeriding is the term people have given to the riders that like to tackle extreme obsticals and highly technical sections of terrain. Freeriders are the guys you see dropping off of 10' ledges and what not.
A freeride bike is typically fairly heavy, and designed to survive large impacts. It usually has 5-8" travel front and back, double crown fork, chain guides, 36+ spoke wheels, large volume tires and extra beefy parts such as the frame, stem, rims, handle bar, ect.
Freeriders do not tend to ride at very fast speeds, so the weight of the bike is less of an issue.
Cheers,
Juba