An article that discusses the differences: Active, Passive, and Semi-Passive RFID Tags: How To Choose?
For passive there’s a reader that sends out a signal. The tag sucks up a little bit of that energy and then spits it back and the reader hears that.
In the middle there is what’s usually called an assisted-passive tag which works just like a passive tag, but has a battery. Instead of just being able to use the energy that it sucks up from the reader, it can steal a little bit from the battery and it can give a louder response. So battery-assisted (assisted-passive) is kind of sitting in there in the middle.
An active tag is actually some kind of smart device. And that kind of bridges into real time locationing where active tags have been popular. But they don’t really do the RFID kind of thing. Instead they identify something as their main goal.