The NFC Data Exchange Format (NDEF) is a binary format that is used for data exchange through NFC. This data format is commonly used in NFC devices (like smartphones) and NFC tags.
NDEF messages consist of one or more records of different types. The type indicates the kind of data that the record contains, and the series of record types in a message defines the message type. For example, a URI message contains one record that encodes a URL string. A launch app message, however, might contain two records that can launch an app on an Android device or a Windows Phone device, respectively.
NDEF message and record format explains in more detail how NDEF messages are built up.
The nRF5 SDK provides modules that help you generate and encode NDEF messages and records. There are modules for specific message and record types as well as a generic generator that you can use to easily implement other standardized records and messages or even create your own records. See Generating messages and records for more information. If you use the provided modules, you do not need deep knowlege of the NDEF specification to start using NFC.
If you want to implement a polling device that reads data from other tags, you can use the provided Message and record parser (in combination with the Adafruit NFC Shield library and the Type 2 Tag parser) to parse the NDEF messages read from the tag.
NDEF message and record format