What is the difference between user_id and account_id in the User object ? Both are different and I'm looking for the unique ID of the User.

{
  "items": [
    {
      "user_id": 2144246,
      "user_type": "registered",
      "creation_date": 1362659682,
      "display_name": "Tony",
      "profile_image": "https://i.sstatic.net/ZEhow.jpg?s=128&g=1",
      "reputation": 68,
      "reputation_change_day": 0,
      "reputation_change_week": 0,
      "reputation_change_month": 15,
      "reputation_change_quarter": 25,
      "reputation_change_year": 67,
      "last_access_date": 1377435062,
      "last_modified_date": 1377018698,
      "is_employee": false,
      "link": "http://stackoverflow.com/users/2144246/tony",
      "website_url": "",
      "account_id": 2417804,
      "badge_counts": {
        "gold": 0,
        "silver": 0,
        "bronze": 5
      }
    }
  ],
  "quota_remaining": 9999,
  "quota_max": 10000,
  "has_more": false
}