Phillips head screwdrivers are one of the two most common screwdriver varieties, along with the flat head screwdriver.

Phillips head screws and their screwdrivers attach via a cross-shaped slot. The slot is deeper in the center than at the edges. This key feature is what makes the Phillips head screwdriver so different than other screwdrivers. If you ever stripped a Phillips head screw accidentally because you torqued a screw too tight, it's not your fault! It's designed to do that!

Phillips head screwdrivers were developed during the development of aluminum-skinned airplanes. When workers used power screwdrivers using previous screw-screwdriver interface designs, all too often the powerful screwdrivers over-torqued the screw, leading to damage of very expensive airplane parts. The Phillips head screwdriver, however, made it difficult to over-torque, as additional torque tended to pull the screwdriver out of the screw slot.

This very same design feature is not desirable for the other 99% of the uses for screws. That's why, even after 200 years of using screws to hold things together, people come up with new, better screw-screwdriver interface designs (Torx, etc.).