Musicians commonly use a capo to raise the pitch of a fretted instrument so they can play in a different key using the same fingerings as playing open (i.e., without a capo). In effect, a capo uses a fret of an instrument to create a new nut at a higher note than the instrument's actual nut.
There are capo designs, but most commercial capos consist of a rubber-covered bar that clamps to the instrument's neck in some way to hold down the strings. Capos come in different sizes and shapes for different instruments and fretboard curvatures. The most relevant mechanical factors that vary by type of capo are ease of use, size, degree of interference with the player's hands, and ability to hold down strings uniformly without affecting tuning.