NFC – Near Field Communication, a communication protocol for very short ranges (a few centimeters).
NFC can be used to read or write NFC tags, to implement mobile payments and to transfer data between two devices – Android Beam feature for example.
Most Android devices support reading and writing NFC tags. Most iOS devices support reading tags and new iOS devices also support writing.
There are all kinds of tag technologies. The simplest one, and the one that is supported by both Android and iOS is named NDEF – NFC Data Exchange Format. It can store strings, URLs and other types of data.
NDEF tags are handled by the OS in Android.
B4A – Reading NDEF tags: https://www.b4x.com/android/forum/threads/reading-ndef-data-from-nfc-tags.14931/#content
B4A – Writing NDEF tags, and low level reading of other technologies: https://www.b4x.com/android/forum/threads/64451
B4A – Communicating between two Android devices with Android Beam: https://www.b4x.com/android/forum/threads/60731/#content
B4i – Reading NDEF tags: https://www.b4x.com/android/forum/threads/nfc-reading-ndef-tags.84784/#post-536918
B4i – Writing NDEF tags: https://www.b4x.com/android/forum/threads/nfcex-class-write-ndef-tags.110726/#content