In an interactive 3D virtual world, the user has control over the camera, which is very unlike a 3D animated scene rendered to video, where the camera is fixed. In the case of 3D animation, the position and control of the camera is vital to creating a natural scene. In an interactive world, it is important that the camera's characteristics are set up correctly, but unless we want to animate the camera to move the user at some point, the user is the 'film' director.

The camera basically defines what you see. It can be moved and controlled either by an animation or by the user. It has a location and a direction.

Parameters that you will probably want to be able to read/control at some point in a virtual world include the pitch, yaw and roll (how the camera is angled in the X, Y and Z axis), and it's location (X,Y,Z position in the world). Other paramters that you might want to control are zoom and the field of view (FOV) angle. Most interactive 3D systems calculate and use an appropriate FOV internally to give the user a natural view of a scene but in some cases you might want to deliberately change the FOV to create a distorted perspective.

Object that are not rendered because they are outside of what the camera "sees" are clipped. Since the rendering algorithm doesn't need to bother to draw clipped objects, it can do it's work much faster. The viewing frustrum is the area between the far clipping plane and the near clipping plane which determines what can be seen in the Z axis. This can have a profound effect on rendering performance but should be used carefully to avoid the user seeing an exagerated popping effect as objects suddenly become close enough to come into view. Many 3D games make extensive use of near and far clipping planes to enable the user to travel around a large 3D environment with relatively few polygons displayed at any given moment but the 3D world needs to be carefully designed for this to work effectively (and it works best if the user is moving around quite quickly while not looking much at the scenery).