WordPress JSON API Plugin

Download & Install the JSON API Plugin
The plugin is available at the WordPress website:

http://wordpress.org/extend/plugins/json-api/

Download and install from the WordPress Admin interface.

Configure the Plugin
The plugin features three main functionality:

Core: Basic methods for pulling standard post/category/tag/etc. data.
Posts: Provides additional functionality with regard to posts, including post creation.
Respond: Provides comment and trackback submission methods.
Most people will only need the “Core” method enabled. Advanced users can have a shot at post and comment management.

Using the Plugin
The plugin has a billion uses and methods, all of which are documented at the following address:

http://wordpress.org/extend/plugins/json-api/other_notes/

A few of the more useful methods include pulling all categories…

/*
url ?json=get_category_index
*/
{
  "status": "ok",
  "count": 3,
  "categories": [
    { ... },
    { ... },
    { ... }
  ]
}//....

…and retrieving all posts in a given category…

/*
url ?json=get_category_posts&slug=mootools
*/
{
  "status": "ok",
  "count": 10,
  "count_total": 79,
  "pages": 7,
  "category": { ... }
  "posts": [
    { ... },
    { ... },
    ...
  ]
}//....

…or just grabbing recent posts:

/*
url ?json=get_recent_posts&count=10
*/
{
  "status": "ok",
  "count": 10,
  "count_total": 79,
  "pages": 7,
  "posts": [
    { ... },
    { ... },
    ...
  ]
}//....

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Taken from: https://davidwalsh.name/wordpress-json-plugin