Common wood screws are made of steel or brass. The screw shank is threaded from the point up, about 2/3rds of its length. The upper third is smooth to the head. Flat head screws are used where the screw must be flush with the surface of the work. Round head screws protrude above the surface, making them easier to withdraw. Oval head screws combine features of flatheads and roundheads: they are partly countersunk in the work, and they protrude slightly above the surface.
Most screw heads have either a single slot across the entire width or a recessed, X-shaped slot called a Phillips head.

Wood screw length is measured form the point to the widest part of the head, see illustrations. Lengths range from ¼-in. to 6-ins. And are graduated by eighths of an inch to 1-in. Screws also vary in body diameter, expressed as a gauge number from 0 (about 1/16-in.) to 24 (about 3/8-in.).

Lower-numbered (thinner) screws are for fastening thin wood or where there
is a danger of splitting. High-numbered screws are used where greater strength
is required.

To drive screws easier, pre-drill pilot holes for them about the same diameter as the threads of the screw. If you are countersinking, pre-drill the pilot hole then countersink it. You can also slide the screw threads over soap or beeswax to make them easier to drive-especially in hardwoods. You can by countersinks for portable electric drills and stationary drills that, at one time, drill the pilot hole, shank the hole, and countersink the hole.