tmux is a terminal multiplexer, like e.g. screen, which gives you a possibility to use multiple virtual terminal session within one real terminal session. tmux-cssh (tmux-cluster-ssh) sets a comfortable and easy to use functionality, clustering and synchronizing virtual tmux-sessions, on top of tmux. No need for a x-server or x-forwarding. tmux-cssh works just with tmux and in an low-level terminal-environment, like most server do. Take a look at the pre-build deb-packages.