All of the websites managed by your iCM installation are known as subsites. A single iCM installation can manage any number of subsites and will always contain at least one. While subsites can share content, more often than not they will be distinct separate websites with their own style, content, security, users, and unique URL.
Subsites are organised into zones. Zones are used by the site authentication service and let you control which users have access to which sites. You could create all of your subsites in one zone, so that a user could log into all of them with the same username and password, or you could segregate them into unique zones so that users would have to register separately with each site.
To access the full subsite management section of iCM, you'll need to be logged into the iCM enterprise URL, which is usually http://my-website.com/enterprise/icm. Once logged in, navigate to the Definitions section of iCM Management and click on "Subsites".
If you are logged into iCM on a subsite URL, and you are a member of the subsite's default user group, you can access the "Properties", "Settings" and "Configuration" settings for the current site, but won't be able to see any other subsites.
For all of your subsite configuration, product settings and theme features, see Subsite Configuration - Framework Features