Informal is between formal and casual. On an informal night I will wear slacks and a jacket (and a tie most of the time). My wife will wear a nice dress or skirt or pants outfit and sandals, usually no hose or stockings...
On casual nights I go for khakis and plo shirts or linen shirts and my ergualr bucks or deck shoes, she is in flowey dresses or pants (khakis) and sandals again.
Hope this helps.