The term “capo” is derived from the Italian words "capo tasto" which means head fret. This makes sense, as a capo's function is essentially to move the first fret anywhere you choose on the fingerboard. Early capo designs that were widely used throughout the 60's and 70's utilized an elastic band that stretched around the neck of the guitar securing a bar over the fretboard. Though these classic elastic band style capos are not widely used anymore, the basic functionality of any capo remains the same: to hold the strings down anywhere on the fretboard.
These days, the most common capos used are built by companies like Shubb, Kyser, Paige, and G7th. Modern capos range from simple in design to more elaborate. The G7th Performance 2 capos for example, have a very sleek, space age appeal, while also being highly functional and easy to use. I recently started using this capo and have been very happy with it so far. One bit of advice I would offer to those of you who are thinking about getting a capo is to avoid the "economy" models, while these will typically save you a few bucks, they are generally not built well and require extra time to put on and take off -- not good if you are in the middle of songs and want to avoid that awkward stage silence! For $20 you can get a great capo that will last you years -- or as long as it takes you to lose it!