There is always a lot of confusion about the difference between Self-Tapping and Self-Drilling screws. And then there is just as much over the difference between Thread-Cutting and Thread-Forming screws. So here is a brief explanation of all of them.
Self-Tapping Screws form threads in the material into which they are installed. Therefore, not as much torque is needed for installing them. A pilot hole is needed prior to installation. As the screw is being inserted, the cutting edges are displacing material within the pilot hole. This can be accomplished in a couple ways and so there are two types of screws falling under the self-tapping category. Tapping screws come in either coarse or fine threads. They can be used in just about all materials and work well in situations where vibration can cause loosening. Self-Tapping screws are also known as sheet metal screws as they are ideal for securing thin sheet metal onto wood or metal frames.
Self-Drilling Screws drill their own hole and make their own threads. They actually look like a drill bit on their end. Many people refer to them as a Tek Screw.
Thread-Cutting Screws are a type of self-tapping screw. This screw requires a pilot hole and works well in wood and metal. Thread Cutting Screws can further be categorized by their type.
Type 1 - work best for harder metals.
Type 23 - work best in softer metals.
Type 25 - are specifically designed for for plastic.
Type F - offer the most secure, fit in vibrating parts and are compatible with most materials.
Thread-Forming Screws move the material around inside the pilot hole and so then sort of lock into the screws threads. These screws work really well in situations requiring a secure fit to resist loosening. No additional lock washers or Loctite are needed. These screws require a bit of torque to get them in.
Thread-Rolling Screws are pretty much the same as thread cutting screws but work better in plastic. They possess a tri-lobular cross-section so when they install, they push the material outwards.