The 180 Degree Rule and Shot Reverse Shot.

The 180 degree rule is the rule in which you keep the camera placed on an imaginary axis around two characters you are filming, not breaking the 180 degree barrier around one side of the two characters, keeping the left character on the left and the right character on the right. When filming a sequence, you must keep to that one side you choose in order to keep continuity in both of the characters eye lines,looking towards one another.

This picture I copied from Flickr demonstrates this rule, the camera keeping in the blue semi circle. The red character stays on the left, and the blue character stay on the right, as if both characters are looking at one another. If the camera then breaks this semi circle, as pictured above, the blue character now appears on the left, and the red character now appears on the right.

Shot reverse shot is the transition of edited shots between two characters having a conversation or engaging in another kind of dialogue. The shots follow the conversation between the characters, keeping to continuity of the eye lines of them both looking to one another.

The diagram above, which I copied from Google images, demonstrates the shot reverse shot rule, straight cutting between two characters as they engage in dialogue.