javascript: What's the difference between functionName() and functionName?
<p>I've noticed that you can reference a function with or without the parentheses. Why?, what's the difference?</p>

<p>As a slight aside, I've noticed this works:
<code>window.onload = functionName;</code></p>

<p>Whereas this doesn't: 
<code>window.onload = functionName();</code></p>

<p>Could anyone explain why the top line of code works and the bottom doesn't?</p>
