What are self-tapping screws and thread forming screws?

A self-tapping screw is a screw that can tap its own hole as it is driven into the material either plastic, wood or metal.  There are two types of self-tapping screws, thread-forming & Thread-Tapping

 Thread-Tapping: These screws do not require a pilot hole but one can be drilled to make fastening easier. Remember to make the pilot hole smaller than the diameter of the thread tapping screw. Thread-tapping screws remove material and create a path for the screw. The drawbacks to using this type of screw is when they are removed it can result in stripped threads. If you experience stripped threads you should use a larger thread-tapping screw when reattachment is made.  

Thread-forming: Before you can use these screws you will need to drill a pilot hole as the screw itself does not contain a pointed tip. Thread-forming screws are usually used to join plastic materials together. Due to their design they typically grip material more tightly. Care should be taken as there is a risk of over-tightening the screw, which can result in joined materials breaking from the pressure. 

Different types of recess, Head and Point of Self-Tapping Screws

6-Lobe	External 12 Point	Phil	Hex Socket	Slot	Hex-Slot	Pozi( Type 1A)	External 6-Lobe

Phil+Slot	Snake Eye	Hex+Pin	Tri-Win	Bugle	Pan	Hex Flange	Flat + Nibs

Trim Hex HD	Hex Washer Face HD.	Dome Head	Oval HD	Round sQ. Neck	Undercut Hex Washer	One Way	Should Bol

⇒  The Sizes of different Recess for your reference

The different Point of Self Tapping Screws