Casual weddings are often held outdoors, such as a beach or backyard. Though this indicates a more laid-back event, jeans, shorts, and tank tops are usually not the right choice unless the invitation says otherwise.
Women might choose a summer sundress is appropriate in a more casual fabric like cotton. Guys should go for slacks or khaki pants with a button-down shirt and tie (tie may be optional). Add a sport jacket or sweater depending on the weather.