Flex uses AS3, just like Flash, so they are the same OOP-wise. Both produce SWFs. The main difference between the two is that Flash is mainly timeline-based, whereas Flex is code-based. Flex’s IDE is FlexBuilder, which is based on Eclipse.
If you’ve been programming for a while, you have probably used Eclipse at some point. It’s a great IDE.
http://www.adobe.com/products/flex/