Active transducers generate electric current or voltage directly in response to environment stimulations.Examples would be thermocouples.Thermocouples produce voltage difference on its terminals ,proportional to the temperature of the environment. Passive transducers produce a change in some passive quantity such as capacitance, resistance or inductance...

Well you might have thousands of definations regards the topic even i have them none of them ever stayed. This is a question & every engineer should know the answer. Sensor reacts to a physical,chemical & biological condition.It senses.It can be considered as a detector. The conversion of Energy from one form to another is called as transduction...