The bow is a very important part of the violin, and can cost a great deal of money. In fact a good bow is almost as important to a violin-player as a good violin.
At one end of the bow, where you hold it, is the 'frog'. As well as being the handle, the frog clamps the bow hairs at this end and has a little metal adjuster which you twist to tighten and loosen them. On the bow in this picture you can also see a piece of rubber around the bow which is used to protect the violin-player's thumb.