Instances exist in a space. An instance is a Java webapp running in a servlet container.
There are two kinds of instance:
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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.