Is it a "best practice" to give a div an ID for manipulation with code or to just assign a class and use it?

If I have a div such as:
<div id="div1" class="div1"></div>
I can access it in JavaScript by both the ID and the class such as:
$('#div1')
or $('.div1) and then manipulate it. Which is a best practices way of dealing with Divs?