Wood screws are probably the most commonly-used mechanical woodworking fasteners. They are used primarily for connecting wood to wood and are renowned for the clamping force that they provide to strengthen a joint. They can also be used to attach hinges, hardware, locks and other non-wood objects.
There are many different types of screws, but only certain types of screws are applicable for woodworking.
While all wood screws have an aggressive thread for grabbing and holding the wood, there are many differences in the varieties of wood screws, based on the gauge, length, head type and drive type of the screw in question.
The two most common head types for wood screws are slotted (commonly referred to as flathead) and Phillips (sometimes referred to as cross-head). These two types have been "industry-standard" for a number of years, but two additional head types have been gaining prominence in woodworking screws of late: square-head and square-head Phillips (a combination of Phillips and square-head). These two screw types tend to grip a bit better and resist stripping more than the traditional types, particularly when driving screws with a cordless drill or power drill.
Most wood screws usually have one of three head shapes. Round head screws have a rounded top but a flat underside and are typically used for affixing thin objects to wood.
Flat head screws fit into tapered recesses (such as the holes in hinges) and will be flush with the surface when properly applied. Finally, oval head screws are a bit of a combination of the previous two, in that they have a tapered bottom (similar to the flat head screws), but also a slightly rounded top.
Wood screws are rated in gauges, which are indicative of their thread diameter. Simply put, the larger the gauge, the thicker the screw. Most home centers will carry wood screws in as small of a gauge as 6 (which is slightly thicker than 1/8-inch), up to a 12 gauge (which is a bit less than 1/4-inch). In the United States, wood screws larger than 12-gauge are typically listed by their imperial sizes, beginning at 1/4-inch.
The last major differentiation between screws is in the length of the screw. Most American lumber yards and home centers will have screws in lengths from 1/2-inch up to 3-inches, depending on the gauge.
Wood screws are usually made of hardened steel but are often found in brass and stainless steel. Some may have coatings to help prevent corrosion, but be aware that those coatings may stain some types of wood.
When using wood screws to affix two pieces of hardwood stock, be certain to pre-drill a pilot hole and countersink. Failure to do so may cause the stock to crack, which will likely cause a big problem that you'll have to deal with.
Also, when choosing screws for connecting two pieces of stock, choose a screw that is as long as possible without poking through the back side of the receiving piece of stock. This will help ensure a strong connection without defacing the receiving piece.
When using a flathead screw, choose a screwdriver whose tip is not wider than the screw, as the screwdriver will end up marring the wood as the screw is set. If the head is too narrow, the screw may strip.