The following KB articles relate to this topic:

Mega Menu

The mega menu offers another way to deliver your top-level navigation by providing more of a column-based layout versus a dropdown/dropout list.

There are some advantages to this option. Sites that have a lot of content can better organize their pages in a more visible way. Users can see all of the subpages listed at once instead of having to hover multiple times.

MegaMenuImage

Mega Menu Examples