A screw is a type of fastener that you use to attach things together with. Wooden furniture is often held together with screws. Screws are threaded so that they are less likely to pull out once they have been tightened, unlike nails. Screws come with different shaped holes in the top, and the ends of screwdrivers come in the same shapes. The commonest ones are flathead or slotted head, and phillips head (cross shaped).