A radio transmitter-receiver that is activated when it receives a predetermined signal: RFID (Radio Frequency Identification) transponders come in many forms, including smart labels, simple tags, smart cards and keychain fobs.

Injectable ID chip or biochip transponders

Biochip transponders are electronic devices that are inserted under the skin of an animal to provide it with a unique identification number.

Injectable ID transponders are less painful, faster to implement, and more cost-effective than ear-tags, brands, or tattoos, have been used to identify livestock animals; such as, pigs, sheep, cows, and horses for several years.

Once inserted under an animal's skin the transponder remains inactive until read by a compatible scanner.

The scanner works by sending a low frequency radio signal to "wake up" the transponder and provide it with the power it needs to send its unique identification code back to the scanner where it is read, compared to other codes in the database so it can positively identify each animal.

Here is a List of Radio Frequency Identification articles for further explanations: RFID