180 Degree Rule Diagram

When filming two characters interacting with each other, the 180 degree rule must always be remembered; Imagine that there is an imaginary line going through the line of sight of both characters. If the camera if filming from the left side, the camera must not cut to the other side of the imaginary line.
  You can however change sides of the characters, but only if the camera films the transition to the other side, but then the camera must stay on that side.

The reason why this rule is in place is to avoid confusion for the audience. As the camera is basically the view point of the audience, cutting to the other side of the line makes the audience think that there could be another character when there actually isn't.

This diagram shows that the 180 degree rule is in place. The black line is the imaginary line between the characters view points. The shot on the left, below the line, shows the yellow character on the left being behind the brown character on the right, with yellow facing away from the camera and brown facing the camera. The shot above the line violates the 180 degree rule, showing yellow on the right, and with brown on the left. This confuses the audience, as the point of view has changed.