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,
Note: I misunderstood your last sentence, Mezmo. No wonder your layman's understanding suited me so well. ;)
To be on the safe side when using a passive preamp, I would want my source to have an output impedance less than 500 ohm, source output voltage at least 2V (this also depends on the input sensitivity of your amp), my amplifier input impedance greater than 50k ohm, and total interconnect length from source to amp not longer than 6 feet. In my system, I have a autoformer passive preamp, USB DAC with output impedance of 50 ohm and output voltage of 2.5 volts, amplifier input impedance of 100k ohm (with input sensitivity of 1.25 or 2.8V depending on the amp), and total interconnect length of about 5 feet. This way I can have all the transparency, naturalness, and openness of a passive without significantly losing dynamics of an active preamp.
Dracule1 said:
source output voltage at least 2V (this also depends on the input sensitivity of your amp)

Can you expand on this?
For example, if amp X has an input sensitivity of 1.25 V, 1.25 V from the source (eg,CD player) is need to achieve full rated power from your amp X. So if amp X is rated at 100 watts, it needs 1.25 V to produce 100 watts. If amp Y has input sensitivity of 2.8 V, then your standard CD player output of 2 V is not enough to get full power out of your amp. In this case you need active preamp, to provide that extra output to get full power out of your amp. Most passive transformer or autoformer preamp can provide an extra 6 dB of gain (stepped resistor, pot, or LDR based passives can't), so you can mitigate some of this. I'm not too certain on about how the length of the interconnects affects this. But I think longer interconnects will decrease the amount of effective volts the amp sees from the source. So I try to keep my interconnects as short as possible which also helps to prevent significantly losing high frequency response. If anyone more technical can find any error in my explaination, please correct me. I'm not that technical.
+1 for the Luminous preamp. It's about $150 with the better caps and they'll use your systems measurements to get the right resistor. You can't beat it for the money.

What am I missing? My amp has a sensitivity of 775mV (VTL compact 100), and I've been running 20 foot interconnects between the amps and various passive pre's with great results. Not sure of the numbers for my dac (Levinson360s). For cables I've used Apogee, Discovery, and Crimson Musiclink. My current, favorite, and last passive is a Bent NOH based on the music first design and transformers. I've also used the Luminous and the Prometheus passives.

Go figure, it sounds great!