The differences between active and passive 3D technology are sometimes confused, but are actually easy to understand.
The differences between active and passive 3D technology has to do with whether information is communicated during the 3D viewing process that creates of change of state required for the viewing process. Currently, the only active technology is liquid crystal shutter glasses, because each side of the glasses alternates state from transparent to opaque in synchronization with a signal from an emitter that receives its signal from a 3D processor. While this happens very quickly, the shuttering effect is due to active communications with the source of the 3D display. Without this communications link, the process would not work.
Passive 3D technology has no communications or synchronization with a movie being displayed in 3D. Both polarized viewing technologies (linear and circular) and anaglyph 3D technology are passive technologies.
Whether the approach to creating the 3D illusion is active or passive, it is necessary to separate a left-eye perspective from a right-eye perspective and display both in a manner that causes the brain to combine the images and create the illusion of depth.