While everyone should read this post about Self Defense Laws, martial artists should benefit more from reading it for the fact that with more power, comes more responsibility. Note that laws vary from state to state (or country to country), but most of them are as follows:
The part which will effect the majority of the people is the non-lethal self defense (since most people don’t go about killing other people), which basically states that the use of physical force is allowed when the victim feels physical force is about to be used on him. The physical force used must be proportional to the attack, ie. you don’t beat the attacker within an inch of his life just because he pushed you. Also, physical force can never be used in response to a word, unless the word is a threat (such as “I am going to kill you”).
The other part of Self Defense Laws is the use of deadly force. Deadly force is justified ONLY when the victim believes deadly force is about to be used on him.
The Self Defense Laws on properties states that you can use non-lethal force only when he believes that such actions are necessary to terminate or prevent another person’s force entry or attack on his property (or dwelling). Deadly force in such an attack is authorized when a violent entry is made and the victim believes it is necessary.
Apart from what is stated above, remember that the true martial artist is strong both in his attacks and in his judgement. Your martial art prowess should never be used to take advantage of other people, or for any other form of abuse.