What exactly does it mean to "work" on a relationship?

Many of my clients understand that relationships take work.  What is often less clear is what exactly is meant by "work" in a relationship.  Sometimes people stay in bad relationships for a long time, believing that they just need to keep "working" at it, when no amount of work is going to salvage the relationship and the smart thing to do is move on.  But what does "work" really mean?

Let's clarify what "work" means in healthy relationships...