Computer architecture

In computer engineering, computer architecture is the conceptual design and fundamental operational structure of a computer system. It is the technical drawings and functional description of all design requirements (especially speeds and interconnections), it is how to design and implement various parts of a computer — focusing largely on the way by which the central processing unit (CPU) operates internally and how it accesses addresses in memory.
It can be defined as the science and art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals.
Computer architecture includes at least three main subcategories:
Once both ISA and microarchitecture has been specified, the actual computing system needs to be designed into hardware. This design process is called "implementation". Implementation is usually a hardware engineering design process.
Implementation can be further broken down into three but not fully separate pieces: 
For CPUs, the entire implementation process is often called CPU design; it can also be a family of related CPU designs, such as RISC and CISC.
More sub-definitions.
Some practitioners of computer architecture use more fine subcategories: