Before embarking on using open source cloud technology for your web property, a basic understanding of cloud, as it’s used in the industry, is essential. While there might be exceptions, here are the definitions.
A software application delivered on the web instead of installing standalone software on a particular OS requiring some environment qualifies as Software as a Service. This makes applications available anywhere on a browser in any OS, device, and also makes latest updates available asynchronously, e.g., email on the web, banking on the web.
PaaS
A web application requires software in turn to be built like the environment, application server, and web server and the configuration of these should be adequate for the applications deployed on this platform. For a Java-based application, one may require a JBoss server to deploy code. With Platform as a Service, instead of trying to manage the JBoss server deployment and configuration, it can be made available as a service and integrated right within your IDE, e.g., openshift.redhat.com, cloudfoundry.org.