A typical flat head manual screwdriver.
A Phillips head screwdriver.
Screwdrivers come in a large variety of sizes to match those of screws, from tiny jeweller's screwdrivers up. It is important to use a screwdriver that is the right size and type for the screw used, or it is likely that the screw will be damaged in the process of inserting it. When tightening a screw with force, it is important to press the head hard into the screw, again to avoid damaging the screw.
Many screwdriver designs have a handle with detachable head (the part of the screwdriver in contact with the screw), allowing a set of one handle and several heads to be used for a variety of screw sizes and types. This kind of design has allowed the development of electrically-powered screwdrivers, which as the name suggests use an electric motor to rotate the bit.
Many contemporary electrical appliances, if they use screws at all, use Torx screws with oddly-shaped heads in an attempt to prevent users of the device disassembling them - see the article on screws for details. However, screwdrivers specifically designed for these special screws have become widely available in electronics stores.