The phillips-head screwdriver is designed to drive the screw of the same name. Phillips-head screwdrivers have cross-shaped tips, with the center of the cross centered on the screw head. The cross shape prevents the screwdriver from sliding sideways out of the head (a common problem for slotted screwdrivers), and keeps the screwdriver centered on the screw.
Phillips-head screwdrivers come in several sizes, denoted by numbers, with lager numbers denoting a larger, heavier cross. Screws and screwdrivers of different sizes either will not work together, or may damage each other if used together. The #2 Phillips is the most common size.