How to update page without reloading?

I have visited some sites where when I select a product to add to my shopping cart, the quantity and total price is updated without the pages just completely reloading. I don't know if that part was in a frame or what but is there any way I can do this? I want to click an item on the page and have another section change. What I'm really aiming for is to click an item and have a thumbnail of the image appear with the rest at the top.

Any suggestions?

Thanks!

Check out the DIV object for Internet Explorer and the LAYER object for Netscape.