Tons of websites have this feature where hovering over a link in the navigation bar extends a dropdown with more details. There are countless such sites,
The Dell and Nvidia homepages are two examples.
I’ve looked it up, and it seems like you have to change the visibility of a list onmouseover and onmouseout.
How can I replicate this effect?
Have a look at DynamicDrive.com. They have loads of menu’s, many with dropdown effects. They all include tutorials on how to implement them.
http://www.dynamicdrive.com/dynamicindex1/index.html