There are a number of different servers or 'worlds', and you can have a maximum of one city on each world. These cities are basically independent, so the main benefit is that you can play more because you are able manage a number of different cities at the same time through the use of numerous web browsers.
However, please note that all of your cities share the same pool of diamonds. So if you buy diamonds you can spend them on any world, and if you earn diamonds in one world then you can also spend them in any of the others.
It is also important to remember that you aren't allowed to set up any cross-world deals with other players, like 'I'll donate to your Statue of Zeus in Arvahall if you donate to my Cathedral of Aachen in Jaims". This is a severe violation of game rules and will result in appropriate action being taken.
