1. Parts, individual part that con not be split.
2. Component, the assembly of parts that perform an action. CNC Machined Components can be either a part or a combination of a number of parts. In this assembly, there is a part that is the main one performs the intended action, and the other parts are only attached, fastened, guided and so on.
As for casting and forging, is the manufacturing methods classification of parts. Different materials and methods of manufacturing, can make parts with different mechanical properties. It is not the same concept with components.