In Rebecca Solomon's 1851 painting The Governess, the title figure (seated right, with her charge) exhibits the modest dress and deportment appropriate to her quasi-invisible role in the Victorian household.
A governess is a woman employed to teach and train children in a private household. In contrast to a nanny (formerly called a nurse), she concentrates on teaching children, rather than caring for their physical needs. Her charges are of school age rather than babies.
The position of governess used to be common in well-off European families before the First World War, especially in the countryside where no suitable school existed nearby. Parents' preference to educate their children at home--rather than send them away to boarding school for months at a time--varied across time and countries. Governesses were usually in charge of girls and younger boys. When a boy was old enough, he left his governess for a tutor or a school.
Governesses are rarer now, except within large and wealthy households or royal families such as the Saudi royal family and in remote regions such as outback Australia. There has been a recent resurgence amongst families worldwide to employ governesses or full-time tutors. The reasons for this include personal security, the benefits of a tailored education, and the flexibility to travel or live in multiple locations.
Traditionally, governesses taught "the three Rs" (reading, writing, and arithmetic)  to young children. They also taught the "accomplishments" expected of middle-class women to the young ladies under their care, such as French or another language, the piano or another musical instrument, and often painting (usually the more ladylike watercolours rather than oils) or poetry. It was also possible for other teachers (usually male) with specialist knowledge and skills to be brought in, such as, a drawing master or dancing master.
In the United Kingdom
The governess occupied a uniquely awkward position in the Victorian household, because she was neither a servant nor yet a member of the host family. She worked in the upper-class home of the landed gentry or aristocracy. She herself had a middle-class background and education, yet was paid for her services. As a sign of this social limbo she frequently ate on her own, away from the rest of the family and servants. By definition, a governess was an unmarried woman who lived in someone else's home, which meant that she was subject to their rules. In any case, she had to maintain an impeccable reputation by avoiding anything which could embarrass or offend her employers. If a particular governess was young and attractive, the lady of the house might well perceive a potential threat to her marriage, and enforce the governess's social exclusion more rigorously. As a result of these various restrictions, the lifestyle of the typical Victorian governess was often one of social isolation and solitude, without the opportunity to make friends. The fact that her presence in the household was underpinned by an employment contract emphasized that she could never truly be part of the host family.
However, being a governess was one of the few legitimate ways by which an unmarried middle class woman could support herself in Victorian society. The majority of governesses were women whose fortunes had drastically declined, due to perhaps the death of their father or both of their parents, or the failure of the family business, and had no relatives willing to take them in. Not surprisingly, her position was often depicted as one to be pitied, and the only way out of it was to get married. It was difficult for a governess to find a suitable husband because most of the eligible men she encountered were her social superiors, who preferred a bride from within their own social class, particularly since such women generally had better financial resources.
Once a governess's charges grew up, she had to seek a new position, or, exceptionally, might be retained by the grown-up daughter as a paid companion.
Outside the United Kingdom
An option for the more adventurous was to find an appointment abroad. The Russian Empire proved to be a relatively well-paid option for many. According to Harvey Pitcher in When Miss Emmie was in Russia: English Governesses before, during and after the October Revolution, as many as thousands of English-speaking governesses went there. As English became the fashionable language of choice among the aristocracy during the later days of the regime, clearly they were displacing opportunities formerly spread more across the French-speaking world. The estimate of numbers ('thousands'), although necessarily vague, is justified by some knowledge of the main lodging house used by those not accommodated with their host families, St. Andrew's House, Moscow, and by the places of worship they preferentially frequented, for example the church associated with the House. Pitcher drew extensively on the archives of the Governesses' Benevolent Institution in London; there is also some allusion to the phenomenon of governesses being engaged abroad in A galaxy of governesses by Bea Howe.
Margaret Pole, Countess of Salisbury, governess to the future Queen Mary I of England. They were also third cousins (Mary's paternal grandmother Elizabeth of York was first cousins with Margaret Pole (their fathers Edward IV and George Plantagenet, 1st Duke of Clarence, 1st Earl of Salisbury, 1st Earl of Warwick KG were brothers).
Jane Austen's novel Emma (1815) opens with the eponymous heroine losing Miss Taylor, the governess who had become a family companion, to marriage with Mr. Weston. Later, Jane Fairfax feels the threat of being forced to become a governess if her covert attachment to Frank Churchill all comes to nothing.
In the Jeanne-Marie Leprince de Beaumont's writing Containing Dialogues between a Governess and Several Young Ladies of Quality Her Scholars, which was published in The Young Misses Magazine, she writes about the polite talks between Ms. Bonne and her class.
Peterson, M. Jeanne: "The Victorian Governess: Status Incongruence in Family and Society", in Suffer and Be Still: Women In the Victorian Age, ed. Martha Vicinus. Bloomington: Indiana University Press, 1972.