If you do a google search for "Platform as a Service", not surprising the Wikipedia definition http://en.wikipedia.org/wiki/Platform_as_a_service gets the top ranking but it is just one definition. On that google results page you will see several definitions, all different.
Also not surprising alongside that listing are some of the major players, Engine Yard, SalesForce.com, Microsoft, Oracle and IBM.
Ok Platform as a Service aka PaaS has one common definition, it is a service for delivery of applications.
To some it is just that...manage the deployment of applications.
For others like WorkXpress, http://www.workxpress.com/ offer not only the ability to deploy applications to the cloud, but a Rapid Application Development environment to actually build the applications.
For others like Heroku, http://www.heroku.com/ offers the deployment and management of applications you develop in multiple languages.
Some like SalesForce.com offer deployment of their applications and yours that extend the SalesForce.com CRM to the cloud.
Still others offer some mix of Infrastructure as a Service (IaaS) and a small number offer Integration as a Service to one degree or another.
Not surprising, we have our own definition of Platform as a Service and that is:
The O4BO Platform that provide a computing platform and a solution stack as a service. Along with software as a service (SaaS) and Infrastructure as a Service (IaaS), and Integration as a Service. We believe a Platform that is designed from the ground up for Small to Medium sized Businesses is not designed for Developers although developers will find it easier to use. A Platform that doesn't include Software as a Service that is fully integrated, is less useful. Software as a Service with just one application is less useful. Infrastructure as a Service with nothing but management of the cloud is less useful. Integration as a Service that isn't integrated with the Infrastructure, Software as a Service applications and the Platform is less useful. A Platform that only works with a proprietary application stack and does not allow you to deploy your own applications is less useful.
The O4BO Platform as a Service, includes SaaS applications in ERP, Accounting, CRM, BI, Analytics, Web Content Management and a Portal. It also includes Infrastructure as a Service with Cloud Management on multiple clouds. It also includes an Enterprise Service Bus that is fully integrated with the Platform, Portal and SaaS applications. It also includes a Dynamic Context Aware Rules Engine to allow for easy customization often with no programming. Any PaaS that doesn't offer all this, is less useful.