An Organization is simply a group of projects: they share the same projects page and some important settings. Organizations are mostly companies that own projects; however you may want to make use of organizations for structuring your projects.
Once you're logged in, on your projects page, open the top-left dropdown to view your Organizations:
To create a new Organization, click "New Organization". Fill in a name and you're done!
Switch back to the newly-created Organization and start creating a project.
The Personal space is your default space: any project created outside of Organizations will be created here. You can switch to it at any time from the top-left dropdown of your project list page.
You may be required to log in again when you switch between Organizations depending of their authentication settings.
then enter the name of your Organization as communicated by your administrator and click "Login":