What is...a SHADOW CATCHER?

Ever since mis-hearing someone who was speaking about "Shadow Casters" (i.e. spot lights), I have been intrigued at the thought of a Shadow Catcher.  Perhaps it is just the mystery of not being able to define what a shadow catcher is/looks like, that has peaked my interest.

Maybe shadow catchers are walls haphazardly placed in the landscape?  Or, maybe they are massive holes...crevasses in the landscape?  Glass fronted buildings facing the sun seem to consume shadows originating from surrounding buildings and sidewalk activity...maybe they are shadow catchers?  Who knows?  Perhaps the eWorld will help me out with this one.  Any ideas?