Garbage collection is an immensely important part of ActionScript programming. You should consider memory management in every ActionScript program you write. If you create an object, you should decide whether that object is needed for the entire lifespan of the program. If not, you should add code that deactivates and then disposes of the object.
For more general information on garbage collection in programming languages, see "Garbage collection (computer science)" in Wikipedia, The Free Encyclopedia at http://en.wikipedia.org/wiki/Garbage_collection_(computer_science), and The Memory Management Reference, at http://www.memorymanagement.org.
For a series of self-published articles on garbage collection in ActionScript 3.0 by Grant Skinner, see http://www.gskinner.com/blog/archives/2006/06/as3_resource_ma.html and http://gskinner.com/talks/resource-management/.
In the next chapter, we'll explore some of ActionScript's less commonly used tools for altering the structure of classes and objects at runtime.