The mistress of the robes was the senior lady in the Royal Household of the United Kingdom, who would, by appointment, attend on the Queen. Queens dowager retained their own mistresses of the robes. In the 18th century Princesses of Wales had one, too.
Sarah, Duchess of Marlborough, mistress of the robes to Queen Anne
A lady-in-waiting or court lady is a female personal assistant at a court, attending on a royal woman or a high-ranking noblewoman. Historically, in Europe, a lady-in-waiting was often a noblewoman but of lower rank than the woman to whom she attended. Although she may either have received a retainer or may not have received compensation for the service she rendered, a lady-in-waiting was considered more of a secretary, courtier, or companion to her mistress than a servant.
Princess Tatiana Alexandrovna Yusupova, a lady-in-waiting of the Imperial Court of Russia
Tang Dynasty court ladies on A Palace Concert painting
Marie Louise of Savoy-Carignan, Princesse de Lamballe was chief lady-in-waiting to Queen Marie Antoinette of France
Walking behind Queen Elizabeth II on a visit to Toronto in 2010 are two of her ladies-in-waiting: Lady Hussey (left) and Lady Farnham (right)