ActionScript 3 lets you easily obtain any instances class name using a new function called getQualifiedClassName (flash.utils.getQualifiedClassName).

ActionScript Code:

var sprite:Sprite = new Sprite();
trace(getQualifiedClassName(sprite)); // "flash.display::Sprite"

You can also use the getQualifiedSuperclassName (flash.utils.getQualifiedSuperclassName) function to find its superclass

ActionScript Code:

trace(getQualifiedSuperclassName(sprite)); // "flash.display::DisplayObjectContainer"

If you want to go backwards, and convert a string into an actual class reference, you can use getDefinitionByName (flash.utils.getDefinitionByName).

ActionScript Code:

trace(getDefinitionByName("flash.display::Sprite")); // [class Sprite]

Also see describeType() (flash.utils.describeType) in the following post...