In the last few years, JSON has taken its place alongside XML as the de facto way to describe API data. Today's leading APIs support JSON in addition to XML, and an increasing number support only JSON.
If you are thinking of building an API, JSON support is critical. Here's why:
Smaller. Smaller is better, especially in the mobile environment, and since JSON doesn't "say" every field name twice like XML does, JSON output is a lot smaller than XML.
Less complicated. JSON is free of namespaces, attributes, multiple "text" nodes, and other complexities of XML. The result is that JSON parsers exist for every language, they're small, and they're fast. Furthermore, if you need to write your own, it's not complicated. The same goes for security -- all that's necessary to prove that a JSON document is valid JSON is a simple regular expression check, which is easily available in nearly every programming environment.
Tools. An increasing number of tools support JSON. JSON support is not ubiquitous yet, but at the rate JSON is gaining it will be soon.
But JSON isn't for everything. Next up: Why XML isn't dead yet!