I hear people occasionally ask whether they need an "Active" or "Passive" volume pedal. Here's the information you need to make that choice. First of all neither pedal is "active", but they have different resistance pots that makes them appropriate for placing after active or passive circuitry. A typical pot in an active pedal is 25K, versus a passive pedal of around 250-500K.
If you are going to put the volume pedal after a guitar with passive pickups then you absolutely need the passive pedal. The passive pedal is more versatile because it can be placed after an active circuit as well. But the opposite is not true because an active pedal will suck the tone out of your passive pickups. However, if you plan to use a passive pedal in an active circuit it's important to know the input impedance of the device you are plugging the volume pedal into. Here's why....
A volume pedal works as a voltage divider. If the input resistance of the device plugged into the output of the pedal is high (like a pedal or amp input) then the sweep of the pedal is mostly dependent on the taper of the pot. If the input resistance is low then the load affects the sweep response. A high resistance volume pedal (i.e. passive) is affected more than a low resistance pedal (i.e. active) when plugged into a low impedance input. Active circuits that run at line level generally have a lower input impedance (in the 10K's of ohms) vs. passive circuits which have high input impedance (in the 100Ks or 1Ms of ohms).
A passive pedal plugged into a low impedance input will have a sweep that kills the volume too fast. That's because as you sweep the pedal the resistance of the pot inside the pedal will rise quickly in relation to the resistance in the load. You can see if you have a 250K passive pedal plugged into a 25K input then the resistance ratio rises quickly when you sweep the pedal. An active pedal of 25K plugged into an input of 25K will sweep much more evenly.
An example of somewhere a passive pedal would not work well is in some amp loops. Some amp loops run at instrument level and some at line level. If your amp runs at line level and has a low return impedance then you will want an active pedal.
Bulgera 333 FX return 470K - OK for passive pedal. Fender Hot Rod Deluxe FX return 57K - Passive pedal will give poor sweep