This is a MiFare Classic keychain fob - often used for electronic locks or customer identification but also found in other systems where a small and easy to carry tag is desired. The tag contains a NXP S50 chip and an antenna, and is passively powered by the reader/writer when placed a couple inches away.
Read more: http://www.elecfreaks.com/store/mifare-classic-s501356mhz-1k-clear-tag-p-480.html#ixzz2yOx8iYHF