WSO2 Stratos is an open-source platform as a service (PaaS) providing a complete middleware environment with support for multi-tenancy, self-service provisioning, and extensive integrated services. It offers solutions for data management, cloud middleware applications, and is built to avoid vendor lock-in with open standards and modular components. Stratos supports elastic scaling, security features, and provides a billing and metering system for resource management across various tenants.