Passive RFID and Active RFID
Passive tags refer to RFID tags which are powered solely by the RFID interrogator. The interrogator emits a radio frequency (RF), which powers the silicon chip on the tag when it is within range of the RF field. When the power to the silicon chip on the tag meets the minimum voltage threshold it require to turn on, the silcion chip can then send back information on the same RF wave. Range is usually limited to several meters.
Active tags refer to RFID tags which have their own power source, so they can recieve a weaker signal from the interrogator (i.e. be further away), and the power source on the tag boosts the return signal. These types can have ranges of many tens of meters and even hundreds of meters, but cost more becasue of their size and sophistication. Battery life can also limit the life of the tag.
Semi Passive tags refer to tags with a power source (usually a laminar, flexible, low cost battery) which can be used for on tag sensing (e.g. temperature), but not to boost range.