Sites are a set of pages that display content and provide access to specific applications. Sites can have members, which are given exclusive access to specific pages or content.
Actually, sites are special groups that have a set of users or a set of user groups. That is, a site can have a set of users associated. Sites are used to represent a set of users who share common interests. An organization can be associated and unassociated to an existing site whenever required for business needs.
In Liferay, the site idea allows the user to perform the task easily and faster using predefined site templates: