JavaScript - With or Without Enclosing Script tag

This is really simple. But to be honest, I am still wondering why the later one will not work?

// Functional

<script type="text/javascript" src="lib/jquery/jquery.js"></script>

// Non Functional

<script type="text/javascript" src="lib/jquery/jquery.js" />

I believe it is nothing to do with JQuery. More to JavaScript thing.