Domination Versus Mode has two teams facing off against each other to hold down three different positions on the map, gaining more victroy points the longer they hold each objective. It comes with a new SubZero map set in the Siberian mountains.
Marathon, meanwhile, is an extended mode which lets players take on the recentco-op campaign in the form of one continuous level. Previously the campaign was divided into three acts, but now you'll have to take the whole thing on at once.
You can find out more about the new update, or head over to the to download the free game client.