ISA is a type of computer architecture with a instruction set
which provides commands to the processor.

The instruction set consists of
Native data types
Instructions
Registers
Addressing modes
Memory architecture
Interrupt and exception handling
External I/O.

The x86 instruction set is common on computers today. Different computer processors can use almost the same instruction set while still having very different internal design. Both the Intel Pentium and AMD Athlon processors use same x86 instruction set. An instruction set can be built into the hardware of the processor, or it can be emulated in software, using an interpreter. The hardware design is more efficient and faster .

A typical instruction consists of two parts: Opcode and Operand.
Opcode or operational code is the instruction applied.
It can be loading data, storing data etc.
Oprand is the memory register or data upon which instruction is applied.

Addressing Modes: Depends on type of instruction applied

Direct mode : straight data is accessed (memory location)
Indirect mode : location of the data is accessed (Using a variable)

In ISA the CPU performance is dependent upon Instruction Count, CPI (Cycles per instruction) and Clock cycle time.