Computer Architecture Subclasses

Instruction Set Architecture (ISA)	is a set of opcodes an ISA specifies the behavior of opcodes running on implementations (i.e. microarchitecture) of that ISA in a fashion that does not depend on the characteristics of that implementation
Computer Organization or Microarchitecture	sometimes abbreviated as µarch or uarch is the way a given instruction set architecture is implemented in a particular processor

Difference Between Instruction Set Architecture & Computer Organization

For example, both Intel and AMD processors have the same x86 architecture (having the same ISA), but how the two companies implement that architecture (i.e. computer organization) is usually very different. The same programs run correctly on both because the architecture is the same, but they may run at different speeds because the organizations are different