If you dont know what this is or what it does don’t ask. Its not interesesting, but its important. Sort of like your spleen.
In windows 7 it can get pretty large – Mine was 11.2gb, significant on a 64gb ssd. This wonderous hack brought it down to around 6gb: http://www.iishacks.com/2011/06/23/reduce-windows-7-winsxs-folder-size/