I think it's movie clips? can u help?

First off, I was not sure what to put in the title because I don’t know how the following thing is done.

Ok, here goes. If you look at the following site, you’ll notice that there is a nice transition as buttons are pressed, example: if you are in the about us page and click on our work, the about us button slides nicely back into position as the our work button falls into place, corresponding movies below also slide in and out.

I can create sites and know how to make buttons etc, If I’D made that site, when a person clicked the our work button, I would NOT be able to make the other button slide back into place but WOULD be able to change pages below without the nice transitions. Am I making sense? I am now ready to learn this skill and hope you can help me out or point me in the right direction towards tutorials


Many thanks for your time.