Maiden is an old-fashioned way to refer to a young, unmarried woman. I think nowadays it's used in poetry and fairy tales rather than in everyday speech.

Maid can be used the same way, but it also refers to a female servant. The general usage in the U.S. these days is to describe a person that is paid to clean homes or offices.

"Maiden" would never be used with the "servant" connotation. 

I hope that helps!

Thanks for your answer to "maiden vs maid"

"Maiden" can (in an old-fashioned way) mean an unmarried woman or even a virgin. Maiden is still used as ad adjective in a metaphorical sense in some phrases - "Maiden voyage", "Maiden over".

a "Maid" is usually young, but isn't necessarily - hence "old maid"!