Computer architecture refers to the design and operational structure of computers. First of all, there is no such thing as standard computer architecture. Usually, computer architecture includes three main categories by default, which are as follows:
Instruction set architecture or ISA, can be defined as an abstract model of a computing system that is seen by a machine language programmer including the instruction set, memory address modes, processor registers, and address and data formats.
Micro-architecture is also known as Computer organization of a lower level. It is a detailed description of sufficient system that describes the operation of all parts of the computing system. How the parts are inter-connected and operate in order to implement the ISA.
System Design refers to all of the other hardware components within a computing system such as system interconnecting components like computer buses and switches, memory controllers, hierarchies and CPU off-load mechanisms such as direct memory access.