Apache Stratos (Incubating) is a polyglot PaaS framework, providing developers a cloud-based environment for developing, testing, and running scalable applications, and IT providers high utilization rates, automated resource management, and platform-wide insight including monitoring and billing. 
Apache Stratos helps run Tomcat, PHP and MySQL apps as a service on all the major cloud infrastructures. It brings self-service management, elastic scaling, multi-tenant deployment, usage monitoring as well as further capabilities. In addition, it brings the ability to take any server software and make it into a Stratos Cartridge. This means any server can be run 'as-a-Service' alongside the other app containers. 
According to the diagram middle layer is the stratos foundation which consists of Elastic Load Balancer, Cloud Controller, Stratos Controller and CLI / WebUI Tool. On top of that there are some services such as Logging Service, Registry Service, Messaging Service and Billing Service. We can configure those services with the top layer. After you configure the foundation layer with this services you can use these services in the run time.
A Cartridge is a package of code or configuration that plugs into Stratos to offer a new PaaS Service. A Cartridge is also a Virtual Machine (VM) image plus configuration and it can operate in two modes that are namely single tenant and multi-tenant.
For more information see Cartridges.