Infrastructure as a service (IaaS) is an instant computing infrastructure, provisioned and managed over the Internet. Quickly scale up and down with demand, and pay only for what you use.
Platform as a service (PaaS) is a complete development and deployment environment in the cloud, with resources that enable you to deliver everything from simple cloud-based apps to sophisticated, cloud-enabled enterprise applications.
Software as a service (SaaS) allows users to connect to and use cloud-based apps over the Internet. SaaS provides a complete software solution that you purchase on a pay-as-you-go basis. Common examples are email, calendaring, and office tools (such as Microsoft Office 365).