Font set to 'menu'

I'm testing various fonts for the grid (to find the one that renders best in current versions of IE, FF and Chrome).

I see that many of the AW examples specify the grid font as 'menu'. This actually renders quite nicely.

Can someone explain what effect this value has i.e. is 'menu' an actual font, or does it mean that some other system-specified font is used? Apologies if I'm missing something obvious.

Also, if no font size is specified, what font size is used as the default (e.g. 11px, or something set in the local computer)?