Generally the DVR You purchase will be set up to record only when it detects movement in the video.
Your system do this digitally when notice the difference in pixels on the screen.
Usually Your system when it notices differences it will be triggered to start recording a few seconds before the movement, and a few seconds later, this is called recording pre-alarm and post-alarm.
DVR systems with Pre and post alarm triggers will also give You the entire event as they will retroactively save video that occurs before the system saw motion and will continue recording for a specified amount of time after the movement stops.
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Most systems have a sensitivity adjustment that will make more or less movement needed before start recording and it may take some adjustments to set up Your system correctly.
