A passive pre-amp takes the output form the source and attenuates it between 100% and 0 (i.e. it only reduces the levelof what comes in - it cannot boost it). For the majority of todays sources, with a reasonable output level that's fine. An active pre-amp can apply an actual gain to the level of the signal coming in.
Passive pre-amps are simpler and theoretically ought to have less of a detrimental effect on the incoming signal, but that's not to say that a well designed active pre amp can't be just as good or better than any passive one.
I had a Creek OBH-12 passive pre in my system for a while, and when i introduced it, it was like a blanket had been taken off from over my speakers (compared with the pre built into the Arcam A32 I had at the time). Have a look for one - they go for £120 ish on ebay last time I looked. If you don't have a big system I reckon that is one of Hifi's biggest bargains.