what is the work of java script function getElementById()

<input id="btnok" type="button" value="button" />
<input id="Button1" type="button" value="button" />
<div id="div1" />
<div id="div2" />
<script type="text/javascript">
 
document.getElementById("btnok").value; //It works on only that element which match the id with "btnok" or you can say      works on unique Id.


document.getElementsByTagName("div").length; //It works on all  that element which have the tag name "div"  
   
   </script>

// where you enter id of element after hashTag
$('#IdOfControl')

// or you can select anything with a certain class :
$('.class1')