Essentially, it's made up of very small turns that happen in pretty much realtime. Every single day something can happen. A player can adjust the speed at which days progress on a scale of 1 to 5 (at will) and can also Pause. Some things are also calculated every month (tech costs, army pay, etc...) and some things are calculated every year (census tax). Hence, in a multiplayer campaign, the host controls the speed at which the days progress (usually 2-3), and any player can Pause if they really need to, or ask the host to slow it down. Hence, everyone plays simultaneously. There's a fairly sizeable multiplayer community, but you'll have to go to the Paradox forums, as these games last a LONG time and are very organised. You can't just drop into the multiplayer lobby and expect a game.