Platform as a Service, PaaS in short, is a cloud computing model which provides the complete development and deployment environment in the cloud. PaaS services include software and infrastructure. In this model, the provider maintains servers, networks, storage, operating system software, and databases. The customer can use it all for a monthly fee which is calculated based on their usage of the services. If needed, the customer can buy more resources.
The most well-known examples of PaaS models are perhaps Windows Azure, Google App Engine, and OpenShift.