The hair on our heads does not grow continuously; instead, it cycles through two stages: the resting phase (telogen) and the growth phase (anogen).  Usually, hair remains in the growth stage for 3-6 years before entering a several month-long resting phase.  Sometimes, however, this process is thrown off balance so that more hairs enter the resting phase than ordinarily should.  This is called telogen effluvium.  Watch this video to better understand the process of hair loss.