An Overflow account allows you to use the Overflow (desktop app and sharing on the cloud).
If you start an account by yourself (downloaded the app and started a trial or purchased directly) you will have an Organization named "<firstname> Organization" created by default for you. You can rename this from the Dashboard.
If you created an Overflow account following an invitation to join an organization, then you'll have access to that organization's shared cloud space.
You can be a member of multiple organizations and switch between them in the Dashboard.