Firstly, what is an alloy? This is a material comprising two or more elements, at least one of them a metal. Alloys achieve better specific characteristics than their constituent parts, and steel is itself an alloy of iron.
Alloy steels are steels in which additional elements have been added to the usual iron, carbon, manganese and silicon that is present in ordinary carbon steels in order to improve their properties and performance. This typically involves better strength and/or hardness and/or ductility and/or corrosion resistance. Several additional elements may be present, and sometimes the desired properties are achieved through a combination of alloying and heat treatment.
There is a vast array of alloy steels developed for very specific applications – such as bearings, gears, shafts, drills, saws, bolts, tyrecord, car bodies, aircraft undercarriages, armour, etc, etc. Alloying elements include bismuth, boron, calcium, chromium, cobalt, lead, nickel, molybdenum, selenium, silicon , sulphur, tellurium, tungsten and vanadium. And for more detail click and see wikipedia entry.