Slotted screws are cut across the entire head. With bugle heads that are counter-sunk (flush or below the wood), a deeper slot significantly weakens the head to the point of snapping off.

The Philips head opening is cast into the center of the head, tapering down, following the tapered shape of a bugle head. Thus, the same amount of metal is there to resist the torque of the screwdriver. The four edges of the bit allows more contact area than the two edges of the traditional slot.

The higher torque then lead to the driver camming out of the slot. Improved drivers (or driver bits) were make with grooves in the bit to provide more bite. This only works with larger screws, like drywall or deck screws, since the slot depth must me greater than the groove spacing.

Hex or Allen heads have six-sided bits, providing more surface area, but still suffering from cam-out and stripping for small screws under moderate torque, especially when the metal used is not hard steel.

For smaller screws, a Torx head uses a driver with 6 points and 12 edges. This provides maximum surface contact in the same sized screw head. You'll find these types of screws in small electronics like cameras, laptops, and hard drives. You'll also find them in auto assembly for the dual purpose of fewer damaged screws, and less tinkering by owners without the proper tools.

Square drives offer less cam-out, more pressure, better wear for the bits. Combination Phillips/square give some of the benefit of square drive, but can still be used with a traditional Phillips driver.

Spanner heads or button heads are used for security purposes...not everyone has those bits, and defeating the screw requires time and obvious damage.

One-way screws...well, they only go "on", not off. Security door on apartments and houses are the most common use...the screws are often 1" wide or larger heads and over 6" in length.

See if you can identify the images by their description. There are many other types. But that could take pages to describe.

http://rsrvd.com/mahalopix/ma-screws.jpg

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