A screw makes attaching two pieces of almost anything supremely easy. Unlike with nails, you can remove and reuse screws with ease. Most are made of steel, brass or aluminum, but you’ll also see specialty screws cast of plastic or nylon.
Types of Screws

A screw gets its name from the drive, or shape in its head. The most common are:

• Slotted (straight slot):
  Used for simple joints, such as attaching a faceplate to an electrical outlet.

• Phillips (+ shape):
  Typically used on appliances, machines, hinges and other hardware.
• Torx (star shape), Robertson (square), Hex screw (six-sided):

These are typically used in building and machine industries because they more effectively use the torque delivered by a power drill or pneumatic tool. These are also less common and are therefore used on some consumer products to discourage people from disassembling them.

Uses and Head Shapes:

Secondary to drive type is the material the screw is made to go into (such as wood, sheet metal or drywall) and the shape of its head. The most common are:

• Flathead or countersunk (wood screw):
  This head has a flat top, allowing it to be driven into a tapered hole with the head set below the wood surface. Screws like these are often used in furniture.
• Pan head:
  Sheet-metal screws often use this type, which has a flat head with slightly rounded sides.
• Round head:
  

This looks almost like a half-circle from the side and is typically found on machine screws used in cars or power equipment.

Sizes

In addition to drive type and intended use, screws are also identified by length in inches as well as size and thread gauge, or how close together the threads are. Threads on wood screws are farther apart than on machine and sheet-metal screws; that's necessary for the screws to bite into the wood and not just act like a drill bit.
How to Choose

First, know what you’re screwing into. Screws are labeled according to their use: Your hardware store will have rows of wood screws, sheet-metal screws, drywall screws and the like. If you’re building a birdhouse, for example, you’ll need wood screws.

Consider the thickness of the material you’re joining. In our birdhouse example, we’ll screw through the face of one 3/4-inch board into the end grain of another. We’ll want to pass through the 3/4-inch board, of course, and at least another 3/4 inch into the second piece of wood to give it a good grip, so we’d shop for wood screws no less than 1 1/2 inches long.

Tips and Advice

• Wood has a tendency to split when you drive a screw into it. Pre-drill a hole slightly narrower than the thickness of the screw’s shaft and drive the screw into it.
• Coat the threads with regular bar soap before driving into wood. The soap acts as a lubricant to reduce friction as the screw drives in — meaning less effort for you.