The underlying language for all Flex applications is ActionScript 3. All the Flex components are written in that language. We used ActionScript 3 in order to make the Flex framework maintainable and extensible.
When you’re creating a business application, however, you don’t always want to use the lowest-level language available in a framework. In Flex, we created a higher-level language, MXML, an XML language that sits on top of ActionScript. Because MXML is a domain-specific language targeting the rapid development of Flex user interfaces, you can quickly become productive with MXML.