Quote Originally Posted by bubba
Nuns are generally called Sister with the exception of the head Nun in an order who is called Mother Superior. That's all I know about it.
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Nuns and Sisters are different mainly in their vows they take.

Nuns are like Priest in that they take the vows for life.
Sisters on the other hand are similiar to Brothers in the fact that they take vows yearly and are under no obligation to retake them if they choose not to.