An instruction can be considered to be a command that has been issued to a computer to perform a particular operation. The instruction format contains various field in them such as:

Operation Code Field: Also known as the op code field, this field is used to specify the operation to be performed for the instruction.
Address Field: This field as its name specifies is used to designate the various addresses such as register address and memory address.
Mode field: This field specifies as to how effective address is derives or how an operand is to perform.

For ex: ADD R0, R1. In this case the ADD is the operand whereas the R1, ans R0 are the address fields.   _{Your Comment}