A bow is the device
string musicians (cellists,
double bassists, violists, and even
violinists) move along the strings of their instrument to produce a sound.
Bows consist of two main parts, the bow, and the hair. The bow part is made usually from wood or plastic, and is capable of being extended or shortened in order to tighten or loosen the bow hair. Bows should never be fully tightened, even when playing, as that will cause the bow to stretch into that tightened position, making the hair go limper and limper. Bows should always be stored in a loosened position to prevent the same problem from happening.
The bow's hair comes from the tails of (most often) female horses. Female horses, because females (excuse my lewdness) "pee out their backsides", which leads to the tail hair becoming pitted and irregular in shape. This makes it so that the hair can absorb the rosin that is rubbed onto it, and allows the hair to make a string vibrate. The pits and irregularities also make it very easy for people to get dirt, grit, and oils into the pits, which the rosin then covers up, and leads to an ugly sound. So don't touch your bow hair with anything but your rosin!
Bows are used by classical musicians all the time, but they are also used in other musical styles such as bluegrass and jazz (thanks in large part to the cross-style capabilities of the double bass). Rock and Roll legend Jimmy Page has also used bows in his music, with a guitar that has a special 'curved' bridge. Very cool.