Skunkbush Sumac, /Rhus trilobata/ Nutt. in the Sumac family, Anacardiaceae. These are the flower clusters (what you see is what you get!). The leaves come along a little later. The fruits (red) are soaked in water to produce a lemonade-like beverage.
Try
http://www.cnr.vt.edu/
dendro/
dendrology/
Syllabus2/
rtrilobata.htm
--Ruth Moorhead