The nursery maid reported to the nurse (what now often is called a nanny) and assisted her in taking care of the children of the employer's family. Her duties included tidying and maintaining the nursery and other supportive chores. It was a junior role for young girls, working under the supervision of the experienced and sometimes older nanny. Many of them also had to wear a uniform. Only very wealthy households, or moderately well-off ones with many children, would employ multiple staff in the nursery. Any household wealthy enough to employ a nursemaid would have a full household staff (butler, housekeeper, cook, etc.).
Everything that a parent ordinarily might do, especially the more onerous tasks, could be turned over to a nursemaid. Feeding very young children and supervising somewhat older children at mealtime, seeing that the children are dressed properly, watching over the children as they play outside, and other such tasks could be left to a nursemaid while the lady of the house concerned herself with other affairs, such as furthering her husband's career.
By reason of her close involvement in most if not all of the daily affairs of the children, including maintaining proper standards of behavior, the nursemaid might easily establish the close kind of a relationship with the children that a mother would herself ordinarily form. In cases where the lady of the household has died, a nursemaid might become even more fully a surrogate mother.