Crab-walks are a body weight exercise in which you sit on the floor. From that position, you lift your butt of the ground, supporting your weight on your hands and feet. In this position, you crawl belly up on all fours.
A lot of kids have probably done this in gym class or just playing around, which causes many people to equate it to silliness. But, walking this way can tax the muscles very quickly, and is used as a legitimate exercise by some trainers.
Bear-crawls are a similar movement done from a prone position that taxes the glutes, hams, and quads to an extent.
I can't properly embed videos on my tablet, but search YouTube for crab-walks and bear-crawls for a visual description.