A crank in its simplest form is an arm attached to a shaft at a right angle that rotates and turns the central shaft producing a repeating circular motion. Often the shaft will be attached to a series of gears, turning one gear and forcing the others to turn, or turning a central axle to turn another series of machines.
A mechanical pencil sharpener utilizes a crank to sharpen your pencil. When you turn the handle of the pencil sharpener you are rotating a shaft that acts as a centralized gear, turning two more gears around it. Those gears have blades attached that rotate around your pencil, sharpening it into a fine point.
A fishing reel is an invention that uses a crank to turn an axle which has a spool of fishing line attached to it. This allows the user to wind the fishing line back into the spool, reeling in the line and possibly whatever fish they've caught. There are many different types of fishing reels, however most of them incorporate some kind of crank.
A bicycle uses cranks to help power it and keep it moving. The pedals are attached to the arm of a crank and instead of powering the crank by hand, this crank is powered by force exerted by the rider's legs and body weight. This crank revolves, turning a large central gear which has a chain wrapped around it. As the chain rotates, it turns the wheel of the bike and moves it along.
Reciprocating engines are engines in which a crank is automatically turned to pump pistons in a quick repetitive burst, generating energy within the engine which is then channeled into the drive shaft, which in turn transfers that energy to the axles. Hand cranks were once required to begin the piston process, but were soon replaced with electronic starters.
A hand-powered car window uses a crank to turn a small series of gears which move a ledge supporting the window up and down.