I've created my clan yesterday (we are only in 2 for now asd). You can create it choosing a name, a message to recruit new players and a logo, that you can create using a 10 layers "program" with pre-made images (at start you have only few simbols, but you can purchase a lot more).
At start you can have 10 people in your clan, but you can pay to unlock spaces for more members. Player that create clan is general and he can choose the role of other players, like captain, ecc... Your clan is rewarded with money every 30 mins (if i remember well) and you can select what percentage of these money go to your clan members. You can also select how much pay everyone that join the clan.
All players can make his fortress (there are a lot of buildings that you can purchase), and there's a fortress for the clan too. You can attack other clans or defend from them, and the fortress is a key factor in this, bur I've not yet tried, so I don't know well how it works.
Players, after level 5, can choose to join a clan, become mercenary (he can play for other clans and receive money, but they are not part of the clans) or create a new clan. You can select to accept all players or you have the control to accept only who you want.
I think is all I've understand of the clan system for now, sorry if is not well wtitten, but I'm not english and I've no time to correct all with google translate now XD