How can I achieve this effect?

http://www.colorz.fr/#!/en/work/

I like the way they show / hide content based on clicking on the thumbnails, I just can’t understand how this is actually done. The "hidden" class that was removed using javascript onclick would not really be viable, since the page loaded the content anyway, and if you had many different divs to show / hide, it would ruin the page load time .

I am sure that there is an easy way to achieve this effect, I just can’t think how to do it in a “clean” way.