Q:

Why alert AnchorElement () alerts the href attribute? 

<a href="url">A link</a>

$.each($('a'), function(index,value){
    alert (value)
});

It will alert : url. 
Why this happens? 

A:

It's because the toString() of the anchor gives the URL.   
Live DEMO
alert calls toString() implicity on objects.   so when you alert an array like:
[1,2,3,4,5,6]

Alerting it will give you:
"1,2,3,4,5,6"

Because the toString() of array is the elements separated by a comma. 
If you encounter this problem while debugging, you should use console.log() instead of alert()

A:

value is HTMLAnchorElement object, HTMLAnchorElement.toString() method return its url attribute. 
alert will use .toString() when display an object.