Screws
Common wood screws are made of steel and normally have a rust-resistant zinc coating. Flathead wood screws (FHWS) are made to be driven flush with the surface. You also can counterbore them into a hole in the surface and cover the screw with either a wood plug or wood putty so the fastener is hidden. Oval-head and roundhead wood screws (RHWS) protrude above the surface for decorative effect or a finished look when fastening metal hardware. You'll also choose from three common screw-head slot types: slotted, Phillips, and square drive. These match different screw-driving tools.
A trim-head screw is a thin, case-hardened, double-lead screw with a small head used in place of a finishing nail when fastening trim. Drywall screws, also called case-hardened steel screws, have a skinny shank and a dull black finish. They're exceptionally tough and are most often used with a power drill-driver. Case-hardened screws come in two thread configurations. One-thread single-leads hold best in softwoods and particleboard. Double-leads have twin threads that bite into hardwoods better.
Hanger screws have a wood-screw thread on one end and a machine-screw thread on the other. This allows you to screw one end into a wall stud, for example, and use a nut and bolt on the other. Both of these fasteners are useful when attaching heavy cabinets to a wall. Lag screws are heavy-duty fasteners that have a threaded shank like a wood screw. They have a hex-head like a machine bolt that allows you to use a wrench, rather than a screwdriver, to apply more torque when tightening them.
Screw sizes are easy to understand. The gauge indicates the size of the shank diameter in a range from #0 (smallest) to #24 (largest). Gauge increases about 1/64 inch in each size increment. Lengths begin at 1/4 inch and extend to 4 inches and longer. Each length comes in three or more gauges. The thinner the wood, the smaller gauge screw you need.