PaaS or Platform as a Service is the middleware for cloud computing.
PaaS represents the software layer that allows applications to manage the infrastructure. A PaaS is essential to a cloud computing deployment. Applications could not find the storage, processing power or bandwidth without a platform that provides these services.
At present, there is no fixed standard for PaaS. A number of companies are competing to establish their own platform. As you might expect, they base their PaaS on established corporate development priorities. Microsoft Azure is based on the .Net platform; Salesforce's Force is built around its CRM database. Among the companies who offer their own PaaS: