Mule ESB is a lightweight, open-source integration framework that allows for quick and easy connection of applications to enable data exchange regardless of technology. It uses a service-oriented architecture and can integrate applications using technologies like JMS, web services, JDBC, and HTTP. Mule ESB is highly scalable and manages interactions between applications transparently whether they are on the same machine or across the internet. It is vendor-neutral so developers are not locked into a specific platform.