Or is it only the tags can be active or passive? When someone says "active RFID reader," does it refer to tags only?"
The term "active" applies only to tags. There are readers that work only with active tags and others that work only with passive tags, but the term "active," in the RFID sense, means that the tag has a power source enabling it to broadcast a signal. When people refer to a "passive UHF RFID reader," they mean a reader that can communicate with passive tags. When they refer to an "active UHF RFID reader," they mean a reader that can communicate with active tags. All readers require a power source.
—Mark Roberti, Founder and Editor, RFID Journal