You may want to take a real video, and then add 3D stuff to it. But if you move the camera, the 3D objects won't fit the real footage, so then... what do you do? The answer is: Camera Tracking. By following points in the real video, Blender will "understand" the perspective, analyse it frame to frame and then recreate the movement of the real camera in 3D!! Once you have the camera motion in Blender... adding 3D objects to your scene will be really easy, and most importantly: they'll fit the real footage!!