Mule ESB is an open-source lightweight Java-based enterprise service bus and integration platform that allows developers to connect applications together quickly and easily to exchange data. It provides key components like service creation and hosting, service mediation, message routing, and data transformation. Mule ESB is a complete integration platform that enables addressing of non-functional requirements such as reliability, high availability, scalability, and enterprise security. It also provides an intuitive visual development environment and instant API connectivity to hundreds of popular applications and services.