Well, Maggie got the low down according to the Demoiselle Club and shared it on her blog, so I'm going to pass it on to you...
"Cocktail": Suit, coat and tie for men, calf/length party dresses for women.
"Dressy casual." Coat and tie for men, church quality, dressy dresses, sundresses, pantsuits for women — also known as "clubhouse" and "chic."
"Summer casual." Think J Crew. Seersucker pants, dresses, skirts, sundresses, Bermuda shorts with polo shirts tucked in, cropped pants, short skirts.