The word nun is a technical term for a cloister religious who live in a monastery.
In Shakespeare’s Hamlet, Act III Scene 1, Hamlet tells Ophelia, “Get thee to a nunnery.” The word “nunnery” is an archaic English term for convent or monastery where nuns lived.
Technically, “nun” refers to religious women who take solemn vows, live in a stable manner, and observe the evangelical counsels of poverty, chastity, and obedience. They wear a distinct habit to designate their particular community. Most nuns are cloistered, that is, secluded from the secular world.
Anyone visiting a monastery of nuns must speak to the sisters behind the grate or through a turn. A turn is a way of transporting material and messages from the outside to the monastery. The chapel of a monastery is also separated. Behind the grate you can find the women religious; in front of it is the sanctuary where Mass is celebrated and then the congregation. You address a nun as “sister.”
Unlike nuns, sisters are women religious who can work among the people and therefore are called active sisters as opposed to contemplative. Some sisters belong to religious orders. Orders were founded before the fourteenth century, and the women in them are in solemn vows that only the pope could dispense.
Women religious founded after the fourteenth century are referred to as communities, congregations, or institutes. Women in these religious establishments are not in solemn vows; rather, they take promises. Promises are usually renewed yearly and therefore could be easily dispensed by the superior of the community or the bishop. These women are addressed as sister, and many communities wear distinct habits to signify their specific religious congregation.
However, after the Second Vatican Council when women religious were asked to update their specific habits, many dropped them altogether in favor of street clothes. These communities also have distinct apostolates, which are spiritual missions such as helping the poor, caring for the sick, teaching the youth, praying for others, etc.