Set screws are sometimes called setscrews, socket set screws, or grub screws. They are a type of threaded fastener, most often used for securing components in place. This generally means affixing two or more loose parts to one another, for example by holding one component tightly against (or inside) a second surface.
The key pinning action of one component against another is what differentiates set screws and grub screws from general-purpose screws. In short, they are used to create compression forces which hold two loose objects firmly together without having to rely on a nut.
Typically, set screws are intended for use in fastening down (setting) parts or components that would otherwise be able to move or slide around relative to one another. The set screw works by passing through a threaded hole in one component and butting up tightly against the surface of a second part on the rear side.
They can perform this setting function thanks to their characteristic flat-ended tips, a key feature of most set screw designs. However, it should be noted that cone-point set screws (sometimes referred to simply as cone screws) do also exist. Cone set screws are designed to achieve a similar sort of fixing function, but with the addition of a pointed tip. This is used to penetrate the underlying surface rather than simply pressing up hard against it.
Set screws are widely used in all manner of engineering, product design and repair applications, often to keep a potentially moving part locked in place. This could involve fixing a component against a baseplate, for example, in various sorts of panel arrays and mechanical assemblies. Common specific examples of set screw use might include affixing spindle cams and handles or securing gears and pulleys to a shaft.