The terms "nun" and "sister" are often used interchangeably. However within Roman Catholicism, there is a difference between the two. Here's a simple summary of the differences.
A Catholic nun is a woman who lives as a contemplative life in a monastery which is usually cloistered (or enclosed) or semi-cloistered. Her ministry and prayer life is centered within and around the monastery for the good of the world. She professes the perpetual solemn vows living a life according to the evangelical counsels of poverty, celibacy, and obedience. Check out the Carmelite Nuns of Baltimore for example.
A Catholic sister is a woman who lives, ministers, and prays within the world. A sister's life is often called "active" or "apostolic" because she is engaged in the works of mercy and other ministries that take the Gospel to others where they are. She professes perpetual simple vows living a life according to the evangelical counsels of poverty, celibacy, and obedience. Check out the IHM Sisters of Monroe, Michigan for example.
Because both nuns and sisters belong to the church life form of Religious Life, they can also be called "women religious."
As you might have noticed, there is a difference in the type of vows, solemn vs. simple. The New Commentary on the Code of Canon Law explains the distinction this way:
"The older religious orders (monastic, canon regulars, mendicants, Jesuits) have perpetual solemn vows, and the more recent apostolic congregations have perpetual simple vows. The chief juridical difference between the two is that religious who profess a solemn vow of poverty renounce ownership of all their temporal goods, whereas religious who profess a simple vow of poverty have a right to retain ownership of their patrimony (an estate, endowment or anything inherited from one's parents or ancestors) but must give up its use and any revenue."
In ordinary conversation, the terms "nun" and "sister" are used interchangeably. Both nuns and sisters are addressed as "Sister."
In popular culture, the term "nun" is often more widely accessible and immediately understood to refer to women who have professed the vows of poverty, chastity, and obedience.