PaaS (Platform as a Service) is a layer that runs on top of cloud platforms. Generally PaaS stacks abstract all the unique features of any cloud platform, allowing a developer just just focus on writing good functional code. The PaaS stack then handles connections to database services, autoscaling, etc (example is CloudFoundry and BOSH).
This abstracts the cloud provider, and sets the foundation for true cloud mobility. This I believe is the future, and will be the path to large scale cloud adoption.