Screws are fixings characterized by a spiral thread running around an internal cylinder with some form of indented head. Screws come in many different sizes both gauge and length and in many different materials usually either bright Zinc plated, electro brass plated, stainless steel, chrome or black japanned. Screws also come with a variety of head types and will accept various screw drivers, pozi drive screws, Philips head screws and slotted screws being the most common but there are dozens of other more specialist heads.
Some screws are not threaded all the way to the head, these are usually wood screws, this is so that the screw cannot be over-tightened and risk cracking the wood. Self tapping screws are slightly harder than normal screws and often have a 'broken' thread near the tip which cuts room for the rest of the thread as it is screwed into place, self tapping screws are great time savers for screwing into softer materials. Coach screws are usually much larger screws with wide threads, coach screws are tightened via an external hexagonal head.