Like a lot of web apps the site has pages where the user interacts with data on a page, completing their action by clicking a button.  For example a list of products that they browse with an ‘Add product’ button at the top:

Controls at the top of the Magento product list page

Often the page that you’re looking at will be long with lots of information on it, or with lots of information for you to fill in, and as you scroll down the page you lose the controls at the top.  What Magento does is: as the controls reach the top of the page (where they would normally disappear), they detach from the page and follow you down as you scroll, staying constantly visible at the top of the page.

Magneto controls follow you down the page as you scroll down

This is a neat application of DHTML, used to make life easier and more intuitive for the user, and not just for the sake of fancy graphics.  We murmured our delight in the office when we saw it…