The Phillips head screwdriver was created and patented by Henry Phillips in the 1930s and was originally used on the 1936 Cadillac. The great thing about it is that unlike the flat head screw (with a single ridge at its tip to slide into a screw with one slot), the Phillips screwdriver is self-centering. Its "X" design won't slip out of the X-slotted screw. Instead, it grips the screw firmly in the center, provided it's the suitable size for the screw.
A Phillips screwdriver has a head with pointed edges in the shape of a cross, which fit neatly into the cross slots of a Phillips screw. You can buy a Phillips screwdriver in five different sizes, ranging from zero to four, with four being the largest. If you like to build things yourself, you'll probably need a few Phillips screwdrivers in various sizes, and having a cordless electric Phillips screwdriver in your tool set can really come in handy. Phillips screw heads allow a tighter fit than a flat head screw, which is why most factories and handymen use them. The screws tend to be lightweight and relatively small.
The trick is to match your screwdriver to the type and size of screws you're using. Your choice of screw depends on the type of job you're doing. Use crosshead screws (which include both Phillips and posidrive screws) for most of your power driving. Use single-slot screws for carpentry and joinery. Use only a Phillips screwdriver for Phillips screws.
If you're not sure which screwdriver to use, be gentle as you begin to drive it. If your screwdriver starts wobbling or slipping out of the screw, you know you've chosen the incorrect type or size screwdriver.