Here are some of the common terms in the bike industry and their meanings. It happens too many times that a word is misinterpreted or not understood. This short glossary should help you make a better informed purchase and, in general, understand what the hell we are talking about ;).

Billet	Billet is a solid piece of metal (aluminum or steel) that is cut and carved/shaped with a CNC machine (see below) to obtain a given shape. A billet engine head is obtained from a "brick" of aluminum and cut to get the fins, valve seats, combustion chamber etc. Billet is an alternative to cast metal (see below).
Polished	Polished and Chromed are often found together. Polishing is the process of "rubbing" a piece of metal until it becomes as shiny as possible. This is usually done on aluminum and stainless steel. Other types of steel will rust over time. Polishing is done by first removing the surface imperfections of the part with different types of abrasives and then using buffing wheels and polishing compounds. A polishing compound is a paste with mild abrasives and other chemicals. Polishing creates a high luster but is not as shiny as chrome plating. On stainless steel the metal becomes almost as shiny as chrome but with a slight yellow shade. Chrome is generally slightly blueish. Polished aluminum is not nearly as shiny as the chromed variant and requires constant maintenance since the metal will tarnish over time.
Forged	Custom wheels are either cast or forged. Forging means that the metal is heated at very high temperature making it soft but not melting it. This soft metal is then pressed into shape with a press able to apply hundreds or thousands of pounds of pressure. The advantage of forging is that the metal retains its molecular integrity since it's not melted. In contrast, cast metal is melted and this can create bubbles embedded in the final product. In the RoadStar engine, which is cast, if you remove the rocker arms from the cylinder heads and look inside the arms' guides, usually you can see several bubbles exposed by drilling the hole for the rocker arms. That would not happen with forged or billet parts.
Cast metal	Casting is an economical process where metal is melted and then poured into a cast. This is a much faster process than machining (billet) or forging, thus creating parts that cost less. Cast doesn't have the same sharp lines of billet parts and cannot be used to create certain shapes. Production engines, both Harley-Davidson and Japanese, are usually made with cast parts.