The crankset is what the bicycle pedals attach to and are what your legs spin in circles as you pedal. On modern mountain bikes, the crankset consists of the crank arms, chainrings (the front cogs) and the axle that connects the two crank arms together. The crankset and its number of chainrings dictate how many front gears a bike has. Modern mountain bikes have one, two or three, with the latest trend being the fewer the better.
The length of crank arms does vary, but not as much as seen with road bikes. Crank length in mountain biking is typically more standard to help with leverage at an average lower riding speed. With this, smaller bikes will often use 170mm crank arms, with medium-sized bikes and up using a 175mm crank length. Downhill bikes and similar will use shorter 165mm cranks for improved ground clearance.