Q:

What is the difference between someFunction() and someFunction in an event ?

In this code : 
    function onHover_(){
        $('p').css('background-color', 'red') ;
    }

$( document ).ready(function() {
    //1    
    $('p').click(onHover_);
    //2
    $('p').click(onHover_());
  });

In the first line the onHover_ function gets executed (as I expected) after I'd click on a <p> tag. In the second line the onHover_() gets exectued right away after the document is ready, meaning it doesn't wait for an click event !
Simply, why ?
Here's a jsFiddle to test the code. 

Found this thread on STO but the result is not what the accepted answer would've prediced. 

A:

onHover_  is a function.
onHover_()  executes or calls the function
So when you assign a handler to a event i.e( click, change) , you want the function to be called at the time the event occurs.
But not at the point when the event is bound.
So in this case
$('p').click(onHover_());

The background color for the p tags is changed immediately , instead it should be happening at the time when you click it. That is happening because of () that follows the function name ( It immediately calls the function  ).
If you are still confused with the syntax
$('p').click(onHover_);

is same as
$('p').click( function() {
     $('p').css('background-color', 'red') ;
});

The callback is an anonymous function which gets executed when the particular event is triggered or performed. 

A:

The first one $('p').click(onHover_); is the right syntax, using function refrence as callback. The second one is wrong as calling function without waiting the handler click to be fired and using returned result (if any) as callback.

