Assuming you've got a webcam hooked up to your computer, all you've got to do is run Motion Detector, set your preferences (Motion Detector can capture still images and/or movies complete with timestamps whenever it detects motion), and hit Start to run your new security webcam. Motion Detector's sensitivity slider lets you determine just how much motion the app needs to detect before it starts capturing images. As you can see in the screenshot, triggering motion is indicated by the red squares.
For its part, the application works very well, it's relatively lightweight, and it's very simple to set up and use. We've always pointed to previously mentioned Yawcam for some quick motion-detection via webcam, and while it offers its own impressive set of tools, including FTP upload, I'd still call Motion Detection easier to use. Either way you go, both are great applications for having a little fun setting up a motion-sensing security webcam.