An end mill is a tool used on a milling machine, a machine found in a metal working shop that is used to remove material from a metal block to make it into a finished part. It is one of the tools used on the milling machine to make a particular type of cut.
End mills come in single ended and double ended varieties. A single ended one has only one useful working end; the other end is just a smooth shank for holding the tool in the tooling holder on the milling machine. A double ended one has two useful working ends. When one end becomes dull and no longer useful, it can be installed such that the opposite end can be used.
These tools are made of carbide steels, with the cutting surfaces sometimes made entirely of carbides. It is also common for end mills to be coated with a very thin layer of titanium nitrate. Titanium nitrate is a gold colored coating that prevents the metal chips being removed from the part from sticking to the tool, reducing its life and the quality of the cut. End mills come in a wide variety of shapes and sizes. To those unfamiliar with metal machining, it will look like an odd drill bit.
While this tool can be used to drill a hole, its function is much larger than that. In many ways, a milling machine with an end mill is very similar to a router in wood working. It can be used to cut blind holes or grooves, bevel edges, cut a flat edge, and make holes in a block of material. It can also be used to make bearing raceways and cylinders. You cannot make a square hole with an end mill, but you can make a square post. When coupled with a milling machine, it is possibly the most versatile machine in the shop.