A screw drive is the system used to turn a screw. At a minimum, it is a feature on the screw that allows for it to be turned. Usually it also involves a mating tool, such as a screwdriver, that is used to turn it. The following heads are categorized based on commonality, with the less common drives being classified as "tamper-resistant".
Most heads come in a range of sizes, typically distinguished by a number, such as "Phillips #00" or "Torx T5". These sizes are not characterized by a linear dimension, but are arbitrary designations in the same sense as a "Size 8" shoe or dress.