The initiation of force is the act of one man initiating force against another, as opposed to retaliatory force. Force includes such acts as murder, theft, threats, and fraud. It is acting against another person without their consent.
The initiation of force is never moral. Man's nature is such that he survives by reason. Survival by reason requires the ability to act on your reason. Force destroys that ability. When you use force against someone, you are destroying their ability to survive by destroying their ability to use reason, and their ability to survive will suffer to the extent that force is used.
When force is introduced into the equation of human relations, survival becomes harder and harder until everyone dies. If there is some parasite living off others, one thing is clear: he needs his victims while they do not need him. If you use force to get what you want, not only do you give him reason to retaliate, but you diminish your own ability to survive by becoming dependent on your victim. Once your victims all die off, you're screwed. In the end, the result of force is death.
If a man uses force against you, he is declaring that he does not want to survive by means of reason. He is telling you that he doesn't recognize your right to exist as an independent individual. According to him, might makes right, and he is just taking his share from the local sucker.
When you come up against a person who views force as the proper means of relating to people, you know that this is a person not worth dealing with. This is a person outside the realm of morality, and once outside, moral conventions and principles have no place. There is only one way to deal with such a person, and that is with retaliatory force.
The difference between the initiation of force and retaliatory force is that retaliatory force is a response to force. It is force meeting force in kind. It is the only proper response to a person who initiates force.
Since force inhibits survival, men can only thrive within a society if they are shielded from the coercion of others. Government is instituted among men to fulfill this function.