A hole saw, or hole cutter, is a great tool for cutting circular holes through fine surfaces. Best for lighter jobs, whereby a regular drill will either cut away to much surface area or simply have too much power to do the job well.
They're great for cutting through surfaces such as drywall, thin woods, soft metals and similar. They're a fine alternative to using a boring bit - which are often quite expensive. Also, a hole saw set won't empty your pocket the same, so they're not only the better option, they're a cheaper one too.
The best hole saws are featured below, all good examples of what a hole cutter should be or do and they cover various categories, such as the best one for drywall, the best metal cutter and so forth.