You play 1 turn and send it to the next player.
It will take ages with lots of players. The 8 human player game will generally move at a turn a day when everybody is on top of it. 1v1 games go much faster, especially in the beginning. In my 1v1 agaiknst Kemal we played 10+ turns the first few days usually in 1-1,5 hour a day. Now our empires are bigger we play about 5-7 turns a day. Although this seems slow, it's quite ok. Those *!@*! deity AI need lots of attention which would make for a lot of waiting online
Also you can play a few turns during daytime. It takes just a few minutes in between work and this keeps the pace going. One great advantage is you don't need everybody online at the same time to play another session. This is a lot of hassle online and will result in a lot of unfinished games.
So IMO PBEM is a great way to play.