Platform as a service is a cloud model in which providers deliver apps over the Internet and host users' hardware and software on their infrastructure.
Private cloud (also called internal cloud) is a marketing term for an enterprise computing architecture that's protected by a firewall. Promotion of the private cloud model is designed to appeal to an organization that 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). (Continued)
A public cloud is one based on the standard cloud computing model, in which a service provider makes resources, such as virtual machines (VMs), applications or storage, available to the general public over the internet.