A screw is a broad category of mechanical fastener with a threaded shaft, designed to screw into a part. This includes wood screws and self-topping screws, which have a tapered shaft with sharp threads designed to cut a mating thread in the part to which they are fastened. It also includes machine screws, which much more closely resemble bolts, but their entire shaft is normally threaded.
Bolts and screws are sometimes delineated by their use. According to these definitions, a bolt passes through unthreaded holes in parts and is secured by a nut while a screw mates with a threaded hole in one of the parts being fastened. In practice this definition is not strictly applied. The term bolt is usually used for a fastener which is not self-tapping and has only part of its shaft threaded. Fasteners which are either self-tapping or which have their entire shaft threaded are normally called screws.
Screws often rely on friction at the threads to remain securely fastened. In the case of a self-tapping screw, radial expansion of the hole causes much of this friction. For machine screws, friction is mostly caused by axial force resulting from torque applied to the head and the subsequent tension in the screw combined with compression of the parts. Machine screws may alternatively, or additionally, use a locking nut or thread-locking adhesive to prevent loosening.