ActionScript is the foundation language you use to build interactive applications within Flash. Since ActionScript first appeared in Flash, there has been a consistent and deliberate move from Macromedia to increase the support of the language from a simple "gluing" language, adding minimal interaction to Flash Movies to a robust and scalable programming language.
Using ActionScript in Flash
ActionScript first appeared as a programming language in Flash 4. But let's forget that. It was a weak attempt at best. Strangely, the potential sparked enough interest that Macromedia realized that ActionScript needed to be raised a notch or two to be able to compete with the big boys.
With Flash MX, ActionScript was improved upon. Now you could, through some clever tricks, begin object-oriented programming (OOP). OOP is the Holy Grail for programming languages and for programmers. It allows a programmer to create bite-sized chunks of code that can be referenced and reused.
The problem with Flash MX is that you need to jump through hoops to get what you need to work on ActionScript to actually work.