Flex
Flex is an open-source framework for building rich internet applications (RIAs) and cross-platform desktop applications, originally developed by Adobe Systems. It uses a combination of MXML (an XML-based markup language) and ActionScript (an ECMAScript-based language) to create applications that run on the Adobe Flash Player or Adobe AIR runtime. It is particularly known for enabling the development of visually rich, interactive user interfaces with data-driven components.
Developers should learn Flex when building enterprise-level web applications that require complex user interfaces, data visualization, or multimedia integration, especially in environments where Flash Player support is available. It is useful for creating dashboards, data entry forms, and interactive charts, as it provides a component-based architecture and robust data binding capabilities. However, its relevance has declined with the deprecation of Flash Player, making it more of a legacy skill for maintaining existing applications.