Motion detection warns you if something happens before its lens. You can configure WapcamServer to Alert you by email, but also to join the pictures involved as attachment, or to send them on an FTP.
Of course, you can store them on your computer.
Motion detection computes two following frames pixel by pixel. For a single pixel, if the difference between the new one and the old one is bigger than the sensibility, this pixel will be marked.
If the number of marked pixels exceeds the threshold number, an alert is automatically sent.