Quick primer on some of the more common types of screwdriver heads for use in various projects. Please note that this is by no means comprehensive, but is designed to explain the more common types in use for woodworking and computers.
Step 1: Slotted
Slotted screws are the simplest type of screw, consisting of a single slot at the head of the screw. Generally not in heavy use in the US, but they are still around.
Flat head screwdrivers and driver bits (which are used for slotted screws) are measured by the width of the blade in inches, eg: 1/16, 1/8, 1/4, 5/64, 3/16, 3/32, 5/32, 9/32.
When discussing slotted screwdriver bits, sometimes the designation "SL" is used.
Step 2: Phillips
The Phillips screw, named after Henry F. Phillips, was purposely designed to "cam out", or have the screwdriver come out after putting too much torque on the screw. This was an intentional engineering point, since at the beginning of the 20th century, machine tools were generating high amounts of torque that were breaking screw heads off. With the Phillips, the extra torque would cause the driver to pop out, preventing the screw head from breaking off and making production line assembly go more quickly. Personally, I am not a big fan of using Phillips for woodwork, but they are in wide use in the US for both wood and computers, so it is important to include them.
Phillips head screwdrivers and bits are denoted by the abbreviation "PH" and the size in numbers, eg: PH #00, PH #0, PH #1, PH #2, PH #3. "PH #00" is small (used for computers), "PH #3" is large.
Step 3: Square Aka "Robertson"
The sizes of square drivers are designated by "SQ", eg: SQ #0, SQ #1, SQ #2, SQ #3. "SQ #0" is the smallest, "SQ #3" is the largest. Often boxes of Square head screws will come with Square drive bits, usually SQ #2.
A common brand of Square head screws is Backer-On. One company that makes Square head screws, Spax, makes screws that can use either a Square head or Phillips driver. These are great to use since they give you a choice (when I use them I always use the Square head portion.)
Step 4: Torx Aka "Star"
Somewhat more expensive than Phillips, but in my mind, it is worth the money, particularly when woodworking. Common brands include GRK, Hillman and Grip-Rite. The sizes of Torx screws and drivers are designated by a "T" and a number, eg: T7, T8, T9, T10, T15, T20, T25, T27, T30, T40. "T7" is the smallest and "T40" is the largest.
Please note that most Torx screws come with the correct size Torx bit in the box, usually a T20 or T15. Another benefit of Torx heads is that since they do not strip, it is easy to back the screw out in case it is necessary.