A capotasto ("capo") is one accessory that any guitarist should have in his or her arsenal. There are even capos made for banjos and mandolins. Guitar capos come in a wide variety, such as those for steel strings on flat fingerboards found on instruments such as acoustic guitars, capos for curved fingerboards found on electric guitars and the like, and capos for classic guitars which have wide fingerboards. When purchasing a capo, be sure to choose one that fits your guitar. Capos press the strings of the instrument in several different ways, such as with screws, springs, or rubber. You should choose the type of capo you find easiest to use. Incidentally, "capotasto" comes from the Italian words for "head" ("capo") and "key" ("tasto").
Note that, from a maintenance point of view, you should keep the capo attached to your instrument for as short a time as possible, as the capo causes the strings to exert a very strong force on the frets when the device is attached. After you are done using the capo, you should remove it right away. Do not place the instrument in its case with the capo attached. Doing so could damage the frets.