180 degree rule:

This rule is a fundamental technique in which you would need to use in the process of film making in order not to confuse your audience.It states that in a scene where two characters act together, they must always have the same left and right relationship as each other. To be able to do this you must imagine a line extending out between them,as long as you are moving your camera anywhere along this 180 degree line you are following the rule, however crossing over the line will not only cause abashment to the viewers but will break the flow of the scene. The actor originally facing right would now appear to be facing the left.
In this youtube clip, the 180 degree rule is explained with an example.
http://www.youtube.com/watch?v=HdyyuqmCW14