At a minimum, a screw drive is a set of shaped cavities and protrusions on the screw head that allows torque to be applied to it.[1][2] Usually, it also involves a mating tool, such as a screwdriver, that is used to turn it. Some of the less-common drives are classified as being "tamper-resistant".
Most heads come in a range of sizes, typically distinguished by a number, such as "Phillips #00".