Platform as a Service (PaaS) provides a computing platform and solution stack as a service. Google App Engine is a PaaS that allows users to build and host web applications in Google's data centers. It supports languages like Java, Python, Go and PHP. Google App Engine provides services like data storage using the datastore, cloud SQL and cloud storage. It also offers services for tasks like sending mail and caching. Applications run in a secure sandbox and are isolated and independent of hardware locations. Google App Engine is suitable when users don't want server management and need scalability, unpredictable traffic, or pay-per-use pricing.