This document discusses JSON (JavaScript Object Notation), its properties as a data format, how it works, and its advantages for use with AJAX. JSON is a lightweight data format that represents simple data types like strings, numbers, and booleans, as well as more complex data structures. It is human-readable and can be easily parsed into a JavaScript object. Compared to XML, JSON is simpler to use with AJAX due to its direct mapping to JavaScript objects and easier element access. However, security concerns arise from its readability on the client-side. The document recommends using JSON for browser-based applications that require medium data transfer.