Workspace is, very simply put, where all the work happens that you do on Toggl Plan. All timelines, users, plans, tasks and milestones are a part of the workspace, as are settings and - important to note - pricing plan.
One user can have, and pay for multiple workspaces. All workspaces can have different users. At the moment, it's not possible to copy a workspace, all of them have to be set up manually.
You can rename and delete all your workspaces under the Settings menu. There, you can also join workspaces you have been invited to. New invitations are indicated by a small red dot on the workspace tab in settings.
Global settings, or the settings that affect all of your workspace (meaning, they are not only visible to the person making the change) are workspace names, everything billing-related, users and their access rights.