Regarding javascript function writing technique

i have different people write function in js different way. here i am giving few sample code. so some one help me to understand why people write function different way and their significance.

 

1)    function foo()

      {

 

      }

 

2) 

   var foo= $(function(){

   }

3)

   foo : function() {    

      }
 

so please see the above code and tell me the  significance of above different approach of writing javascript code. looking for details discussion with sample code. thanks