Passive pickups use a magnetic source of energy and relatively large coils of wire to generate their signal. Active pickups use smaller coils of wire and have a preamp built into the pickup to boost the signal. The upside to active pickups is that the smaller coils in them make active pickups far less susceptible to outside interference and as a result they are very quiet. Also, with active pickups the output and EQ of the pickup can be changed and shaped by changing the parameters of the preamp. All active pickups operate off of a battery to power the preamp.