difference between an active and a passive preamp?

I have a nad c272 amp and am looking for a good preamp to go with it, but I am on a very tight budget. I see lots of preamps that are acive and some passive - I have no idea of the difference? I have quad 22L speakers and listen to cd only. Any help understanding these differences would be great. I just want simple 2 channel preamp, with as tube like sound as possible. Please help, and many thanks,
And, as a result of having no active gain, passives are generally more transparent, and provide more bang for the buck if gain is not an issue. If you have only one source, check out Luminous audio axiom
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King Rex preamp under $400. As transparent as a passive with tube like mids and deep bass of the solid state pre it is. I believe it has 15 db of gain. A real fine sounding unit the sounds well above its price.