That'll give you a div with "one" inside. Once you click the link, it will change to "two". The getElementById refers to the ID of the div, and the return false; prevents the browser from following the link.
Is this what you are looking for? There are more elegant ways of doing this, and I'd much rather provide a proper example.
Your question is not clear... Any how - from my understanding... Using CSS alone, you cannot change text content of a div with a link. Instead you can give a feel of the change by hidding and showing two different div tags.
Hmm, the way of patila might work, if i didn't have flash-buttons... -_-' Maybe it 's very important, but my menu is made in flash And I would like more something like kesav is saying, hiding and showing buttons.. Doesn't have to be with CSS alone.. But of course, it's made in Flash, extra difficulty