Imperia Online is a MMORTS. You own an "empire" you get each 1-2 minutes new supplies of wood and other resources even if you are not online. When you get online you can upgrade your buildings to generate more amounts of resources and also build barracks etc to get a bigger army. When you get a bigger army you can pillage other empires to gain even more resources or you can even conquer other players cities.
Guy 1: I'm playing Imperia Online, it's a web browsed MMORTS (explains like above)
Guy 2: That sounds exactly like Travian except that Travian is better and it has more features.