What is a private cloud?

Private cloud (also called internal cloud or corporate cloud) is a term for a proprietary computing architecture that provides hosted services to a limited number of people behind a firewall.  Advances in virtualization and distributed computing have allowed corporate network and datacenter administrators to effectively become service providers that meet the needs of their "customers" within the corporation. The words "private cloud" is designed to appeal to an organization that needs or wants more control over their data than they can get by using a third-party hosted service such as Amazon's Elastic Compute Cloud (EC2) or Simple Storage Service (S3). 

The concept of a Private Computer Utility was first described by Douglas Parkhill in his 1966 book "The Challenge of the Computer Utility". The idea was based upon direct comparison with other industries (e.g. the electricity industry) and the extensive use of hybrid supply models to balance and mitigate risks (wikipedia) 

The comparison between private and public cloud is given below.

  

Source: Gartner