Oh, I thought you said mums; never mind.....
Nuns are pretty much the female equivalent to monks: An order which seeks to devote itself to God instead of "worldly things," living together in a monestary or convent away from the cares of the world. In their view, this includes separating themselves from marriage, and instead being "married to God," as it were. The closest Jewish equivalent that I can think of off the top of my head are the Essenes, though I don't think they forbade marriage.
The difference between monks and nuns is primarily that monks are ordained priests, authorized to carry out all of the sacraments of the Roman Catholic faith, while nuns, being women, are not.
Oh, I thought you said mums; never mind.....