Adobe Flex is a Software Development Kit to develop rich internet applications based on the adobe flash platform.Using Flex we can develop desktop and web applications with help of Adobe Flex Compiler and Flash builder.
If you are new to flex then use following links to understand about flex.I hope this will be useful for beginners.
Tour De Flex
Tour de flex is a desktop application it contains more 450 samples with source code so we can easily understands flex capabilities,about components,adobe air,effects,skins and more.It is also available for web version check here.
Flex.org
Flex.org is a dedicated site for flex.It contains flex related resources like training videos,downloads,flex learning tutorials,cookbooks.
Flex Style Explorer
Using Flex style Explorer you can design flex applications online.And you can also try Component Explorer.