A nun is a member of a religious community of women, typically one living under vows of poverty, chastity, and obedience. She may have decided to dedicate her life to serving all other living beings, or she might be an ascetic who voluntarily chose to leave mainstream society and live her life in prayer and contemplation in a monastery or convent.
A "nun" being a religious woman who lives a contemplative and cloistered life of meditation and prayer for the salvation of others, while a "religious sister", in religious institutes like Mother Teresa's Missionaries of Charity, lives an active vocation of both prayer and service, often to the needy, ill, poor, and uneducated.