Checking If Passwords Match On Registration?

So, for a simple little example of what I want to do...

REGISTRATION FORM
<br>
Desired Username: <input type="text" name="username">

[Code]....

I want to rig up something that will automatically check the values posted in "password" and "passwordcheck" before the form is submitted making sure that they match each other and give a little error message if they don't. Ideally, if they don't match it would also prevent the form from even being submitted. My knowledge of Javascript is pretty basic, in fact it primarily consists of what's covered under the "basic" category in [URL] Javascript tutorial, as well as a very modest understanding of form validation in the "advanced" section of the same site. So I haven't had much luck trying to accomplish this myself.

I don't need anything especially fancy (since if it were it would likely be beyond my comprehension at the moment) I just need it to be functional.