The camera's spatial orientation is described by a triple of mutually perpendicular unit vectors. In memory of happy days at the beach, these vectors are called sea, sky, and eye. The screen plane is parallel to the sea-sky plane; the sea vector points horizontally to the right, sky points vertically upward. The eye is their cross product, which points directly at the viewer.
The sea-sky-eye basis is located at the target, so the target is the origin of the screen plane. The viewpoint lies on the line through the target in the direction of the eye vector, and is the center of projection for the default lens. The distance from the viewpoint to the target is the range. The orientation, viewpoint, target, and range completely (and redundantly) determine the camera's geometric situation in the world.