How to have a menu like this (don't know how to describe it. Example:amazon )

I want to google this but just have no idea what to search for. Maybe because my English is not good enough....

I wanna have a menu like the one on homepage of amazon.com (Please see attachment). The function I like is when the mouse is moved to the menu, the next level menu will slide out.
This is probably not related to this theme. But I just hope someone can tell me what is this and what to look for.

Thanks