About Callback method jquery in order to control the animation and functions performed

About animation and has a function to perform control of the following two pieces of code can be well explained,

$("button").click(function(){
  $("p").hide("slow",function(){
    alert("The paragraph is now hidden");
  });
});

After clicking to animate, after the trigger function.

The following code, it is the first implementation of the code, after the trigger animations

$("button").click(function(){
  $("p").hide(1000);
  alert("The paragraph is now hidden");
});