The frames per second (fps) relates to how many pictures the DVR will record in a second. Real-time recording is about 30 fps on each camera. To calculate the fps per camera take the total fps in the system and divide it by the number of video inputs.
For example, a 60 fps digital video recorder with 4 video inputs would result in about 15 fps per camera. The technology has finally gotten to the point now where real-time recording is affordable. If you are recording cash registers or something similar then you should definitely invest in real time recording.