From Team 449 Wiki
A drill bit is placed in the chuck of a drill press or hand drill in order to drill a hole. Drill bits can be classified as twist drill bits, Forstner bits, spade bits, or hole saws. Each drill bit style is used for creating a different type of hole.
Twist drill bits are the most common type of drill bit. They are used to drill bolt holes and large holes when a hole saw bit is not available for that size. They work by cutting the metal and removing it upwards via the twisty part, as in an auger or Archimedian screw. They come in a wide range of sizes, from a hundreth of an inch to an inch and a half. It is not recommended to use twist drill bits for any hole larger than one inch in diameter.
Hole saws are saw shaped bits that make circular holes. They are most useful for making bearing holes, as a thin-wall hole saw is much quicker than upsizing drill bits or using a Christmas tree. Wall thickness is important: the thicker the wall, the slower the speed of drilling and the more oil is needed. Hole saws only remove the outline of the hole, and do not turn the rest of the hole into shavings.
Spade bits work by scraping away material inside the hole. Because of this, they should not be used on metal, as the friction will create a lot of heat and probably jam the drill. Spade bits are much better than twist drill bits at drilling larger holes.
Forstner bits are a combination of a spade bit and a hole saw. As with spade bits, they should not be used on metal if possible. Forstner bits are much like hole saws, except they turn the entire hole into shavings, not just the outline of it.
A Christmas tree (also called a unibit or step drill bit) is shaped like a Christmas tree, and has a series of increasingly wider drill bits. Using a Christmas tree removes the need to use separate bits to upsize holes, but also means that there needs to be another piece of wood or metal below the piece being drilled, as the Christmas tree will drill through eventually, and it is not going to go on the same axis as the other hole. Therefore, if precise holes are needed in something like aluminum tube, don't use a Christmas tree.