There are basically three kinds of evening attires: Casual, Business-Casual, and Formal.
Casual attire would consist of dresses/slacks and blouses for ladies, and sport shirts/trousers for gentlemen.
Business-Casual would include dresses/pantsuits for the ladies, jackets/blazers for gentlemen.
Formal would be long/short cocktail dresses for ladies, dark suits/tuxedos for men. This being said, some cruise lines are very casual and will allow much more casual attire.