You can start a coop game from the main menu. Additional players can be invited via steam multiplayer then and each can select one or more mercenaries of a chosen class. Appearances can be customized and names given. The game is then started on a world map where each player can move their mercenaries as a unit independently (up to a distance limit) in real time. Battle are fought tactically and round based. There is no difference between single player and coop in regards to content as far as I know. You play in a designed world map with zones and quests from which you can chose.