From NES Hacker Wiki
In the example below, JMP is the opcode, $1000 is the operand, and the two combined is the instruction.
0001:4C 00 10 JMP $1000 ; Moves the program counter to $1000.
In this example, DEX is the opcode, but there is no operand since it is implied from the opcode.
0001:CA DEX ; Decrements the X index register.