The FDA has two classes of mouth rinses: therapeutic and cosmetic. Therapeutic mouth rinses (containing fluoride) have generally been shown to help fight plaque, cavities, and gingivitis. Cosmetic mouth rinses simply remove food particles from the mouth, treat bad breath, and/or reduce bacteria, without doing anything to prevent or treat gingivitis.
Patients who have a hard time brushing their teeth (i.e. due to physical problems such as arthritis) can use a therapeutic mouth rinse to help maintain their oral health. However, mouth rinses that claim to treat cavities and plaque are only moderately effective. It is still just as important to regularly rinse with water and use a high-quality fluoride tooth paste when brushing.