The screw is one of the six basic types of simple machines -- it's an inclined plane wrapped around a cylindrical shaft. The inclined plane -- or threads -- on a screw translate a rotational force known as torque into a forward or backward motion. That means by turning a screw, you can lift a weight. You can also use screws to fasten two objects together. In general, screws are better at holding objects together than nails. They're typically easier to remove than nails, too.
There are many different kinds of screws and screwdrivers. The two most common types of screwdrivers are flathead and Philips head screwdrivers. A flathead screwdriver resembles a chisel -- it fits into a screw that has a single slot in the head. A Philips head screwdriver has a tip that looks like a plus sign (+). Many builders prefer Philips head screws to slotted screws.
Using a manual screwdriver often takes a lot of time and effort. Several tool manufacturers offer electric screwdrivers, and most power drills have special bits designed to fit screws. Either way, no toolbox is complete without at least one screwdriver.