With the exception of a few professions (banking, financial services, and other more regulated businesses), casual dressing has become the norm in offices, replacing formal business wear. Women no longer have to wear dresses or skirt suits in most offices. Men don't typically wear suits, unless they are called to a special meeting or make a presentation.
When you are asked to dress in casual attire, avoid the urge to be a sloppy dresser. You want to look polished, even if your attire isn't business formal. Rather than wear acid-washed jeans and a faded T-shirt, slip into some dark wash jeans and a stylish top if you are a woman. Men can wear jeans or khakis and a button-front or golf shirt unless otherwise noted. Make sure all attire is pressed and free from wrinkles.
You may add a cardigan or pullover sweater, lightweight jacket or windbreaker, or a blazer. Sweatpants are never appropriate for anything but working out or lounging around the house. Footwear should be clean and polished – a pair of ballet flats for women and loafers for men. Athletic shoes will work for an ultra casual event, as long as they are clean and without holes. A nice pair of polished boots is also appropriate.