What is sumac used for?
Sumac is commonly used a tart flavoring element in the eastern Mediterranen region.  Historians believe that it was the common sour element in cooking for that area prior to domestication of lemons.  It is a characteristic seasoning element in Fatoush, a Syrian bread salad.

It's not usually available in the mainstream spice section of most grocery stores.  The best place to locate it is in mid-eastern markets, delis, upscale stores such as Dean &amp; Deluca or online through Penzey's or the Spice House.