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Do you mean to say how do I use this stuff?

You can find the documentation here or there's a Get Started section at the right of this page.

For example if you want to see a list of users you could send a web request to http://api.stackoverflow.com/1.0/users and parse then JSON response.

Now if you want to find out details about a particular user lets say your details http://stackoverflow.com/users/566610/jared

Do you see the number 566610 thats the id. Every question, answer, comment ,or user has a id.

So here you need to send your web request to http://api.stackoverflow.com/1.0/users/566610

Your JSON response :

{
  "total": 1,
  "page": 1,
  "pagesize": 30,
  "users": [
    {
      "user_id": 566610,
      "user_type": "registered",
      "creation_date": 1294388578,
      "display_name": "Jared",
      "reputation": 28,
      "email_hash": "e51f7fdd3c09fc9246278abf89f1881e",
      "age": 19,
      "last_access_date": 1294538702,
      "website_url": "http://wphax.com",
      "location": "Wisconsin, US",
      "about_me": "<p>Freelance WordPress Expert</p>\n",
      "question_count": 2,
      "answer_count": 2,
      "view_count": 1,
      "up_vote_count": 0,
      "down_vote_count": 0,
      "association_id": "d0249d62-3c97-4b50-9df1-8c7f377846f6",
      "user_questions_url": "/users/566610/questions",
      "user_answers_url": "/users/566610/answers",
      "user_favorites_url": "/users/566610/favorites",
      "user_tags_url": "/users/566610/tags",
      "user_badges_url": "/users/566610/badges",
      "user_timeline_url": "/users/566610/timeline",
      "user_mentioned_url": "/users/566610/mentioned",
      "user_comments_url": "/users/566610/comments",
      "user_reputation_url": "/users/566610/reputation",
      "badge_counts": {
        "gold": 0,
        "silver": 0,
        "bronze": 4
      }
    }
  ]
}

Take one more example. Lets assume we want to find out list of questions for this you should send your web request to http://api.stackoverflow.com/1.0/questions

and if you want questions related to a particular tag for example wordpress

your url will be http://api.stackoverflow.com/1.0/questions?tagged=wordpress

And once you have finished with your application you can register for an API key.

Read How API Keys Work for more details.

Edit :

You can use StackAPI which is an interactive way to explore the API of the stack family of sites.

I hope this helps.