Wood, Sheet Metal and Drywall Screws -differences, uses, and head typesWood Screws: These have a coarser pitch (few threads per inch) than sheet metal or machine screws, and often have an unthreaded shank. The threadless shank allows the top piece of wood to be pulled flush against the under piece without getting caught on the threads. Some wood screws are tapered from tip to head, also. This site lists pre-drill sizes for various sized screws.
Sheet Metal Screws:Usually threaded all the way to their head, these will work in wood, but wood screws shouldn’t be used in metal (this is based on hardware store employee advice, not experimental evidence). Most of these screws are self-tapping in that they only require a pre-drilled hole (pre-drill sizes), but some come with self-drilling (shown in above pic) or self-tapping tips. Here’sa large list of different types of tips, the more common ones appear to be A, AB (pointed) and B (no point). Here’sa good guide to the different point types and uses. See more pics of thread cutting screws here. Even moregood pictures of different head types.
Drywall Screws:The coarse thread version is meant to secure drywall to wood while the fine thread version is for attachment to metal studs (commonly used in office construction). The head-to-shaft junction is more curved than in a wood screw to prevent tearing of the dry-wall. These can also come with self-drilling tips for wholesale screw suppliers.
Slotted, Phillips and Square drives: The main drawback of slotted heads is that power driven screw drivers easily cam out. Phillips heads address this problem to a certain extent, but these were actually designed to cause the bit to cam out at a certain point to prevent over-tightening. There have been revisions of the original Phillips head, most notably the patented Pozidriv, which does not have rounded internal corners and won’t cause the driving bit to pop out. The square or Robertson drive is least likely to cam out and transfers the greatest amount of torque. The wiki Screwpage and this one describe some other less commong drives.