function($)

function($)

Hey I am a beginner in jquery and I have found these in many places
  1. function($){
                        // what is dollar sign doing here.?
}

      2. $ . fn .myfunction   = function{
            // what is fn here and y not to use directly
            function myfunction(){}?
}      

   3.
            $.myvariable={
                  init://some code,
                  error://some code,
                  title://some code
            // I have seen such thing in $.ajax() function..Is this a way of object declaration.Is "myvariable" an object here?
      }

  Please help