There are a few projects out there [0][1] that make it fairly simple to set up a motion detection capable solution with a simple USB webcam. I'm currently waiting on a cheap infrared camera from China ($4!) for use with my Pi, but I've done testing with an older Logitech HD camera and have had excellent results.
[0] Motion: http://lavrsen.dk/foswiki/bin/view/Motion/WebHome
[1] MotionEye: https://github.com/ccrisan/motioneye