The number of variables that affect the specifications of a simple screw lead to a huge amount of different screws types. Every combination of screw material, coating, head shape, screw drive type, length, gauge and thread size works best for a specific job.
Picking the right screw for the job
Practically anyone who has manually driven a screw into a piece of wood has experienced "camming out," when the screwdriver doesn't have a good grip on the screw head and slips out. This slippage was a driving force in the evolution of screw construction.
That's how we ended up with so many different kinds of screw heads:
Different Types of Screw Heads
Slotted: This is your basic single-groove drive type that you adjust with a slotted screwdriver. Because of the greater chance of slippage, slot heads are best used in low-torque situations, where something needs to be held in place but not tightly secured.
Phillips: The familiar X-shaped Phillips head provides more surface area for a driver to grip than does a slot head. Ironically, this type of screw head was designed to prevent over-tightening by causing the driver to slip out at high torque levels.
Combination slotted/Phillips: A good screw head to use if the tightness of the joint needs to be adjusted. Use a Phillips driver for initial installation and then loosen or tighten the screw with a slotted screwdriver.
Robertson (square): A square Robertson screw head transfers the most amount of torque with the least chance of slippage. To avoid the risk of over-tightening, drive Robertson screws with a drill that has its own adjustable torque setting.
Hex socket: Hex socket heads have a hexagonal slot that can be manipulated with a hex drill bit or an Allen wrench.
Star-shaped: More often known by the proprietary name Torx, star-shaped screw drives usually have six "points" in the slot. Torx screws provide the increased torque transfer of Phillips screws, but can lessen user fatigue because of the reduced need to bear down on the drive tool to keep the drill head from camming out.
To keep installations safe and secure, a number of security heads have been developed. Some more common security screws are available from your local hardware store:
Security pin screws: Some security is attained by the addition of a pin into the center of a hex, Robertson or Torx slot. Unless you have a bit with the complementary hole to fit that pin, you cannot tighten or loosen the screw.
Spanner head: Spanner head screws, also called "snake eyes," have a flat top with two round indentations the same distance from the center. You must have a spanner driver in order to use or adjust this type of security screw.
One-way screws: One-way screws are often used to assemble public restroom stalls. They are similar to standard slotted heads, but the surface that a screwdriver or bit would grip to loosen the screw has been stripped away, so the screw can be tightened but not loosened or removed.
Breakaway heads: In some extreme security situations, screws with breakaway heads are used. Either the screw head is designed to twist off when the driver reaches a certain torque or the head is hammered or chiseled off after the screw is installed.
These are just the more common types of screw heads — the type you'll be able to find at a hardware store. Many other styles have been developed. Some of them — such as Apple Computers' pentalobular head — are proprietary constructions used only by specific companies or in certain industries.