Adobe Flex is similar to Flash in that both of them use the same graphics engine and they both allow you to use Actionscript.
However, Flex is free, open source, and is written using MXML. It consists of Actionscrupt libraries that give access to the Flash drawing engine and a compiler, however, rather than compiling through the development environment such as in flash, it is done via the command line. Flash is also based more around the time line than Flex.
Flex has similarities to HTML and to my understanding is based on XML, so most of it seems fairly easy to read to me. The Actionscript files are virtually the same as the appearance of the Actionscript files you would use in Flash.