Faithfulness is when 2 people have a relationship only with each other. They are committed to each other and if they are having sex they have it only with each other-there are no outside sex/relationship partners. Being unfaithful is when a partner goes outside the relationship for emotion/sex with another person. This is the biggest cause of the spread of HIV/AIDS and other Sexually Transmitted Infections (STIs) today. Go for tests if someone was unfaithful to you.
Attribution: Straight Talk Foundation