What is the difference between a liar and a pathological liar?

A plain old liar: Can lie for many reasons - especially to stay out of trouble, to make them self look good, for some type of gain. A pathological liar on the other hand - can lie about things that don't matter. There doesn't need to be any "gain" from lying. They can lie so much that they forget what lies they told. They can lose track of when they are lying ; when they are telling the truth.