ActionScript is Flash's built-in object-oriented scripting language, it's part of the default package. Like JavaScript, it is based upon Ecmascript and has a component model and can be extended with external libraries and components. But there's a ton of stuff already built-in; especially Flex has almost everything that you need to build business applications.
I've never used the free development tools myself (Flash Pro and Flex Builder here), but Adobe have pretty much opened the entire platform and from what I've read, it should be possible to develop for the Flash platform exclusively with free tools. In any case, Adobe's Flex Builder SDK -does- have all the required tools except for an IDE. If you're more into coding than designing, then the SDK and your text editor of choice should do for you.
I've never used the free development tools myself (Flash Pro and Flex Builder here), but Adobe have pretty much opened the entire platform and from what I've read, it should be possible to develop for the Flash platform exclusively with free tools. In any case, Adobe's Flex Builder SDK -does- have all the required tools except for an IDE. If you're more into coding than designing, then the SDK and your text editor of choice should do for you.