Need help with links; can't say exactly what, I'm clueless..

Hi! I'm wanting to do something, and I don't even know what it's called... So here it is:

I want to setup a link(s) on my homepage, to where when the mouse pointer is over that link, a small "sub menu" of links will automatically appear. An example would be:

Heading link = "Conversion Tables"
Submenu items= -Length
-Temperature
-Volume

I don't think the "Conversion Tables" would actually be a link... (don't know) but when the mouse pointer came into contact with it, a submenu containing the Length, Temp, and Volume links would appear.

As I said, I don't even know what this type of effect would be called... Any help at all would be great!

Thanks!