Allen screws also are commonly known by two other names: hex screws and socket screws. The most distinct part about an Allen screw is the hexagonal hole in the head that can only be loosened and tightened with an Allen/hex/socket wrench or key.
The various types of Allen screws are the hex-cap screw, socket-cap screw (ridged and smooth), the socket flat-head screw, the socket button-head screw and the socket-set screw.
Allen screws, as with many other types of screws, are differentiated by the styles of their heads, which come in all variations ranging from flat-head to button-head to headless. Each head is different so as to serve its respective purpose.
Hex Cap Screw
A hex-cap screw has a cap screw and is used in instances where a tighter screw than what is offered by a hex bolt is necessary. It varies in length, has an Allen drive and can be made from alloy steel, aluminum, or stainless steel.
Socket Cap Screw
There are two variations of the socket cap screw, one in which the sides are ridged and one where the sides are smooth (only the smooth variation is specified as the ridged variation is pretty standard). They have a low head which is small and cylindrical.
Their heads make them especially convenient for projects where a traditional socket could not be used because of space. Typically, they are made from alloy steel and stainless steel and have an Allen or socket drive.
Socket Flat Head Screw
A socket flat head screw has the classic characteristics of a flat head, which is countersunk and has either a flat or rounded edge around the head. It has an Allen or socket drive and can be made of such materials as alloy steel, grade 8 chrome and stainless steel.
Socket Button-Head Screw and Socket-Set Screw
A socket button-head screw is an Allen screw with a low-profile head with the rounded top (as characterized by the button head) and an Allen drive.
A socket set screw varies from the rest of the Allen screws in that it is headless. It is designed that way so that it can be screwed all the way into a threaded hole to provide support.
It has an Allen drive and can be made of alloy steel.