From the Tirerack website:
Forging uses intense heat and pressure to transform a [solid} slug of alloy material into the final shape of a wheel. Forged aluminium is about 300 per cent stronger than cast aluminium, yet less material is needed to produce the same "cast alloy wheel", which results in a lighter product. Because of the basic limitations inherent in forging, most forged wheels are two or three piece units. In two-piece construction, a centre is forged and welded or bolted into a spun or stamped outer rim. In a three-piece wheel, the centre is bolted to an inner and an outer rim half. This stands as an advantage of being easily customisable for a variety of widths and offsets.
Casting is a relatively inexpensive way to produce a high-quality, fairly strong alloy wheel. There are two methods used. One, system is known as gravity casting... whereby the molten material is poured into a mold and allowed to cool. These molds are usually made by machining a piece of material on CNC machine equipments to produce a wheel that only requires minor finishing (like drilling or possibly trimming of some excess metal) to be considered complete. The other and better system used is the low pressure or negative pressure casting. Here instead of pouring the molten material into the mould, the molten alloy is drawn up into the mould using a high-pressure vacuum. This eliminates much of the trapped air found in gravity casting process, producing a stronger wheel that is less porous than a gravity-cast one.
