For some people, the solemn, floor-length, black-and-white habit brings repressed nightmares involving rulers and bleeding knuckles flooding into the conscious mind. Others regard these women as repressed and living in a very unnatural state. As with everything that is even slightly mysterious, there is much misunderstanding when it comes to the real story behind Catholic nuns.
Nuns, first and foremost, devote themselves to a life of service and spirituality away from the outside world. They are actually the female equivalent of a monk. The vast majority of nuns are Catholic; however, there are several Protestant churches (particularly Episcopal) that have orders of nuns. The term "nun" is generic, and can refer to either nuns (who live a completely cloistered existence) or to sisters (who work within a parish). All nuns must take vows of chastity, obedience, and poverty.
Nuns, properly so called, have taken solemn vows and received a papal enclosure. They will never venture out and cannot receive visitors at their convent, under pain of excommunication. Sisters, on the other hand, have taken simple vows and commit themselves to work within the diocese or abroad on missions.
Nuns may be purely contemplative, relegating them to the status of "prayer warriors"; these orders are strictly enclosed. Others combine contemplation with works of charity or foreign missions. Some orders focus on the education of young girls. And still others dedicate their lives to caring for the sick, poor, mentally challenged, and elderly.
The Catholic Encyclopedia claims that women were the first to embrace the religious life for its own sake. The Scriptures cited are 1 Timothy 5:9 and 1 Corinthians 7. These passages do not speak about nuns specifically; the former is in a context of taking care of widows, while the latter is an exhortation to remain content with the life God has given us. Perhaps the best scriptural foundation for the position of "nun" is 1 Corinthians 7:34, "...an unmarried woman or virgin is concerned about the Lord’s affairs: Her aim is to be devoted to the Lord in both body and spirit. But a married woman is concerned about the affairs of this world — how she can please her husband." There is some truth to the idea that an unmarried woman can more faithfully serve the Lord.
There is nothing in Scripture that explicitly forbids the practice of nuns. However, there is also nothing that explicitly describes any similar practice. So, perhaps this is one of those cases when one should look at the motives of the women involved. Many women have felt a call from God to give up all worldly possessions (poverty), remain chaste (chastity), and submit to the authority of the church (obedience). Is there any good reason to assume that this calling did not come from God? Generally, the enemy does not call people into a life of serving others and of prayer. Without a reason to speculate otherwise, perhaps there are many nuns who are genuinely serving God precisely as He has called them to. At the same time, any nun who believes that serving as a nun merits salvation is terribly mistaken. We are saved by faith, receiving Christ as Savior, not by acts of charity, poverty, or obedience.