JavaScript: calling a function with out parenthesis return's whole function as string

I create a javascript Object as below code:

var obj={
  a : 10,
  b : 20,
  add : function(){
     return this.a + this.b;
  },
};

i executed the function with this code obj.add and it return the whole function as string like this:-

function(){
     return this.a + this.b;

but later i try by call the function again including the parenthesis like obj.add() and it return the value 30. Here i couldn't figure out why i get such different out put upon calling the function with obj.add and obj.add()? What is the main difference between calling an objects function with parenthesis and with out parenthesis Can any body help??