A thread is a ridge of uniform section in the form of a helix on the internal or external surface of a cylinder (IFI description) or it could be described as a sloping plane curled around a cylinder.

External threads are on bolts or screws.

Internal threads are on nuts.

There are many forms of threads but two types are in common use on fasteners.

Machine Screw Threads - used on bolts, setscrews, machine screws and designed to mate with preformed threads in nuts or tapped holes.

Exceptions may be thread forming screws like Taptite or self-drilling screws like Teksor thread cutters like Type 23's, which form or cut their own machine screw thread.

Spaced Threads - used on woodscrews, self-tapping screws, coach screws and Type 25 thread cutters. Designed to form its own thread, usually in a pre-drilled hole.

Exceptions may be self piercing screws such as needle points or self-drilling screws like Type 17's which create their own hole; some Teksmay also have spaced threads.