Instances exist in a space . An instance is a Java webapp running in a servlet container.
There are two kinds of instance:
Author instance - Author instances are where editors work. This instance typically resides in a secure location behind a corporate firewall, inaccessible from the Internet. The author instance publishes content to one or more public instances.
Public instance - Public instances receive content from an author instance and serves it to visitors on the Web. No authoring occurs here. This instance resides in a publicly reachable location. You can have more than one public instance serving the same or different content. In a typical deployment you have at least two public instances.
Read more about instances.