Friendship or Relationship?

What is the difference between a friendship and a relationship? There must be some kind of distinction, but why I find this so complicated to define, I'm not so sure. Friendship seems more casual, hanging out together, no obligations to express any deeper feelings. But a relationship can mean one of two things; having relations with someone, just as friends, or the romantically-charged aspect of being a couple. More affection and lust over someone else? The boundary of friendship sure is a fine line that causes mixed feelings and interpretations.