RFID is a Radio Frequency Identification Device.
Two real or potential benefits of RFID are the use of passive RFID's in pets and active RFID's in transit systems. Personally, I have had the former implanted in my cat(s) and felt better knowing that in the event that Negrita went out and lost her way back (impossible..) that she might be returned safely. The latter is a matter of convenience-anything that speeds up already congested Bay Bridge traffic, and the like, is fine by me.
Two privacy concerns related to RFID's are data leakage - aggregated data-knowledge of personal (as a consumer and otherwise) behavior and the possibility of stalking/tracking are both extremely undesirable.