And how do the tags communicate with an RFID reader?
High-frequency (HF) tags communicate with a reader using something called inductive coupling. While ultrahigh-frequency (UHF) tags reflect back a signal using backscatter, HF tags have a coiled antenna that forms an electromagnetic field with an HF reader's coiled antenna. Changes in this field enable the tag and interrogator to communicate. The sizes of the coils in the reader and the tag influence read range, so a very tiny tag with a small coil, for example, would reduce the range. If you have a very large reader antenna, then that can increase the read range—but generally speaking, HF tags operate within 3 feet of a reader antenna.
—Mark Roberti, Founder and Editor, RFID Journal