The data center is the point where you can control your user data. At this moment, is isn't that big, but we will extend it in the next time.
You can see all datatypes of your system. That could be user, pages, images,...
If you click on a type, you will see the description and properties of it.
You can also share your data with other systems. Just click on "New Share". The name is to remember, who gots your data. The api key is a password, to protect the share. Remeber: It's better to take a long key here.
As result you got a link to a xml document, which contains your data.
It's also possible to generate forms for the datatypes. Have a look at form management.