A barbering tool, these days powered by electricity, used in men's haircutting.
Hair used to be cut for the longest time with razors and shears. However, at the beginning of the 20th century some enterprising young men decided to repurpose a veterinary horse hair clipper. It consisted of a set of scissors type handles which connected to a body with a moving comblike blade and a stationary comblike blade. It "chewed" through hair, leaving it all one very nice and even length.
The tool was eventually scaled down and refined for use on human hair, and its adoption led to a revolution in men's hair styling. Now not only was a short haircut feasible with a lot less labor, but by "rocking" the tool from its body onto its heel as you moved up the head from the nape of the neck, you could taper a haircut. (Modern parlance calls that a "fade").
Some enterprising soul decided to make it into an electric tool and the 1920s opened the field to any of a number of electric clipper companies, and the close-faded haircuts of the 1950s overtook the long, slicked-back looks of the 1940s. Still on the same principle: there's a moving blade and a stationary blade, only the two are vibrated at fast speeds by an electric motor.
There are different types in terms of technology: the strongest motors are the rotary ones, like the Oster 76, a classic in the barbering trade. That will plow through hair like a hot knife through butter, and the motor is damn near bulletproof. The other forms of motor are the magnetic motor, which vibrates one blade back and forth by energizing and de-energizing a magnet balanced against a spring (similar to a tattoo machine) and is typically used in high speed, quiet and cool running haircutting, and the pivot motor, which offers far more power but works also by moving the blade back and forth on a pivot.
The clippers can also vary in terms of their usage pattern. A clipper proper is used to cut hair to short lengths evenly and quickly - whether an Oster with a replaceable blade (which can be pulled off and replaced while the clipper is still running) or a magnetic unit with combs locked over the blades so that less hair is able to reach the blades. These have numbers corresponding to the length of hair they leave. A 1 leaves 1/8" of hair, and a 4 leaves 5/8" of hair. Using combs beyond that is typically unweildy, especially around the ears, but you will have the occasional customer who insists on using a #7 or #8.
There are specialized clippers usually referred to as liners, whose sole function is to make hard edges by clipping the hair clean off wherever it touches. It's typically used around ears to terminate the hair above and around the ear, used in a carving motion around the ear itself. It's also used to "block" hair at the back, producing a clean edge above the shirt collar. Turned over, it can be used to clean the hair almost to the skin, and should only be dragged across skin upside down. A straight razor is used in many shops to finish the hair on the neck proper. Some clippers extend their reach beyond the clipper body, referred to as a T-liner, and are used to get very close and precise. Liners are also used to cut lines or other patterns into the hair, especially in African American hairstyles.
The length of the remaining hair is determined either by the blade installed, the comb mounted over the blade, a combination of the two, or in some clippers, a lever on the side whose purpose is to move the blade up and down with respect to the stationary blade, allowing for infinite intermediate lengths between two clipper blades or combs. The lever set up is ideal for fades and tapers, though some cut stair steps with different blades and fade them manually by clipper over comb.
Clipper blades do need sharpening occasionally, and they also need to be "set" so that the moving blade doesn't extend beyond the stationary blade (otherwise what you have is an electric meat knife) but not so far back that close cutting can't be achieved. The blades are typically put together against a hard surface and then screwed down with retaining screws.