The map is divided in regions and provinces, where each province contains aprroximately 3 (the number varies from province to province:2,3, or even 4) regions.
Each region has a city, whose capture assures the conquest of the region.
However, one of the three (2 or 4) cities is also the capital of the province:
The capital allows the construction of a great variety of buildings, it is siegable and also you can watch its growth on the campaign map.
In contrary, the cities, which are not capitals, are not siegable (when you attack them, a field battle occurs, with the actual city depicted far away, like ETW, for example) and have a small limit of available buildings (for example, in city A you can build only farms, while in the city B you can build only ports and navy facilities).
The reason for the low number of sieges is that they wanted, according to CA's statements, to prevent Rome II from becoming a siege fest (like Medieval II) and, therefore, from offering a boring gameplay experience.
The reason for the region-province system is,
in my personal opinion, that they wanted to simplify the campaign management, in order to emphasize in battles and, consequently, attract a larger audience.
I hope I was helpful.
EDIT: Second again! Dog soldier beat me...
