my dog bites!

Question
My name is leanne and my dog bit comeone today.I had him and my other dog out in the yard and they were barking which they always do. I think they are just very nervous dogs. A lady came over and pet my dog hoping that he would stop barking... and he bit her pretty bad. She isn't mad but I am. Is there anything i can do to help my dog? or is it too late? this isn't the first time but this was by far the worst.

Answer
Your dog has already demonstrated active aggression toward a human, by your own report: "this isn't the first time but this was by far the worst."

Your dogs are apparently confined on leash or someone would not have been able to enter your yard and "pet" one.  Confining dogs on chains is a sure way to create aggression.  I have no idea what sort of dogs these are, you've given me absolutely no information.  You need to locate a certified applied animal behaviorist.  It is unethical and unprofessional for anyone to offer remedy for active dog to human aggression in this venue.  Find a behaviorist in your area at the following site:
http://www.animalbehavior.org/ABSAppliedBehavior/caab-directory