At first, a recall of important point: to send a content from/to Android (or other NFC-Forum compliant), the content (URL, vCard, Text, etc.) should be encoded in a NDEF¹ format.
During the communication between 2 devices, they poll for NFCIP connexion. Once connected, the software layer is ensured by LLCP². LLCP is a specification that offers a service-based protocol that could be compared to the network socket-system basics : one service (ie. in network FTP, in NFC SNEP³ or NPP⁴) affected to one port (ie. in network 21 for FTP, in NFC 4 for SNEP).
When LLCP communication is established, one of the devices will ask for SNEP service on Android 4.x.x and for NPP service on Android 2.3.x on the target.
With this short explanation, you should oriented to understand basics, for further details you should read according specifications.
 NDEF: NFC Data Exchange Format
 LLCP: Logical Link Control Protocol
 SNEP: Simple NDEF Exchange Protocol
 NPP: Android NDEF Push Protocol