This is an html tag which creates popup dialog box on a web page.It represents interactive component like window.
Following is an example of Html dialog box:

<div>  
<dialog id="myFirstDialog" style="width:50%;background-color:#F4FFEF;border:1px dotted black;">  
<p><q>I am Pranav Chhabra  working in Evon Technology dehradun 
</q> - <cite>Sachin joshi</cite></p>  
<button id="hide">Close</button>  
</dialog>  
<button id="show">Show Dialog</button>  
</div>  


<script type="text/JavaScript">  
(function() {    
    var dialog = document.getElementById('myFirstDialog');    
    document.getElementById('show').onclick = function() {    
        dialog.show();    
    };    
    document.getElementById('hide').onclick = function() {    
        dialog.close();    
    };    
})();   
</script>