180° Rule

The 180° rule is a basic guideline in film making that states that two characters in the same scene should always have the same left/right relationship to each other. If the camera passes over the imaginary line connecting the two subjects, it is called crossing the line. The new shot, from the opposite side, is known as a reverse angle.

In the example of a dialogue, if Owen (orange shirt in the diagram) is on the left and Bob (blue shirt) is on the right, then Owen should be facing right at all times, even when Bob is off the edge of the frame, and Bob should always be facing left. Shifting to the other side of the characters on a cut, so that Bob is now on the left side and Owen is on the right, will disorient the viewer, and break the flow of the scene.

http://en.wikipedia.org/wiki/180_degree_rule