The term "B" movie comes from a time in Hollywood when movie stars were catagorized as "A" list, "B" list. (Eventually, when Television appeared, TV actors were added to the catagorization in a status "well below" the "B" list actors.) "B Movies," then, were the productions the studios were less proud of and put less money and effort into. Particularly after the advent of Television, "B" movies became the type Terri described above. Generally considered not to be serious cinema, "B" movies usually involved exploitation of some sort -- teenage (though usually played by people in their 20's) rebellion, alien invasions, robots, and small towns being over-run by some sort of terror. (I could go on and on and on about this, but I don't want to bore anyone.)