NFC does not only share the same frequency band (13.56 MHz) but it is based on HF-RFID standards.
Specifically, the NFC interface protocol standard (ISO/IEC 18092 / NFCIP-1) uses components from the RFID standards ISO/IEC 14443 (Type A) and JIS X 6319-4. This essentially merges the technologies MIFARE (by NXP) and FeliCa (by Sony). In addition ISO/IEC 21481 (NFCIP-2) defines compatibility/coexistence mechanisms between NFC and other HF-RFID standards (such as ISO/IEC 15693)
NFCIP-1 defines two communication modes: active and passive NFC peer-to-peer mode at tree different communication speeds (106kbps, 212kbps and 424kbps).
- Passive P2P mode at 106kbps uses the same modulation, coding, framing, and anticollision primitives as ISO/IEC 14443A. One side (one device) operates in a mode that is similar to a ISO/IEC 14443A reader and the other side (other device) operates in a mode that is similar to a ISO/IEC 14443A card.
- Passive P2P mode at 212kbps and 424kbps uses the same modulation, coding, framing, and anticollision primitives as JIS X 6319-4. One side (one device) operates in a mode that is similar to a JIS X 6319-4 reader and the other side (other device) operates in a mode that is similar to a JIS X 6319-4 card.
- Active P2P mode at 106kbps uses the same modulation, coding, and framing(?) as the reader-side of ISO/IEC 14443A.
- Active P2P mode at 212kbps and 424kbps uses the same modulation, coding, and framing (?) as the reader-side of JIS X 6319-4.
This means that NFC devices that support passive P2P mode also support all the protocol primitives to operate as ISO/IEC 14443A and JIS X 6319-4 readers as well as cards. In fact, an NFC device that waits to be activated by another NFC device in passive peer-to-peer mode will also be detectable by an HF-RFID reader (that polls for tags of the respective standard).
Beyond that, the NFC Forum Analog and Digital Protocol specifications also define a reader/writer mode and card-emulation mode that more or less supports various other HF-RFID standards.
In practice, NFC phones typically support at least access to some ISO/IEC 14443 transponders and FeliCa (JIS X 6319-4) cards. The reason for this is that all NFC tags are essentially RFID memory tags based on these "HF-RFID" standards.
Android NFC phones can typically detect and read at least transponders that implement the anti-collision and activation of ISO/IEC 14443-3 (though there are some limitations with Type B), Topaz (thats's a variation of ISO/IEC 14443A), FeliCa (JIS X 6319-4) cards, and ISO/IEC 15693 transponders. Some also support MIFARE Classic cards (which use a protocol similar to ISO/IEC 14443-3A) and B' (a variation of ISO/IEC 14443B). Recent Android NFC devices are also capable of emulating smartcards based on ISO/IEC 14443-4 (typically Type A).
However, one needs to keep in mind that the antennas (and the HF power-supply in general) in NFC smartphones are usually designed to work with small, low-power NFC tags. This often results in bad performance with contactless smartcards (e.g. insufficient energy transfer to power up the card or to perform certain crypto operations, tag-to-reader signal not picked up by the NFC phone, etc.)