It's hierarchical. At the top we have an Instruction. An instruction has a micro-program that defines it's logic. A micro-program consists of micro-instructions in it. Each micro-instruction specify micro-operation(s) for that particular micro-instruction. This is theoretical interpretation.
On the implementation side, a micro-instruction is a control word (Series of 0s and 1s. Each 0 or 1 specify the state of some flag or port or anything and control signal). In a control word, a part of it is control signal which actually specifies the micro-instructions to be carried out (In terms of binary string).
Read this doc