A type of drill that lets you make very big holes. There are two main types - metal and wood. The wood hole saw is a flimsy affair. Basically, it's a drill bit with a skirt around it. The skirt has sharp teeth. The metal hole saw is a vicious thing made of very hard metal. It too has a drill bit in the middle and a skirt with teeth. But it is very solidly made and hardened. When you use hole saws, they usually get quite hot. The metal ones require a powerful electric drill and can be quite expensive.