It's a group stage type used in StarCraft II tournaments like WCS, GSL and GSTL.
It eleminates ties within a Group by being a mixture of group stage and double elimination. See http://wiki.teamliquid.net/starcraft2/User:Capiston/sandbox8 for more information about how it works.
With normal Round Robin, it's nearly impossible to do a group stage in a StarCraft II tournament as many ties in the group are possible ( and likely ) which can throw off the whole schedule. That's why there's seldom a group stage ... and if so, it's a dual tournament.