The technology of Near Field Communication is really interesting. NFC operates in two modes.
Active - Active Mode: In this mode, both devices are generating their own RF fields. This would be in the case that two mobile devices are being used to exchange data.
Active - Passive Mode: In this mode, one of the devices generates the RF field and the other device uses the field to power itself and communicate. The active device is usually the "reader" and the passive device is the "tag".