The mother of the bride should be very smartly dressed, usually in a dress, to or just below the knee, with matching suit jacket. Very dark colours are usually avoided, etiquette dictates no female guest wears white.
The mother of the bride should generally wear whatever she feels comfortable in, whether it be a long dress or one that hits around the knees. Of course, the mother should never outshine the bride in color or style, and should remain age-appropriate.
Any style is acceptable as long as the wearer looks good, and the bride is comfortable with her mother wearing that dress. It is personal preference. Some wear pant suits, and others wear formal gowns. It should be something less revealing than the bride's dress.