AppleScript's chief purpose is to let you communicate with scriptable applications. How you target a scriptable application using AppleScript depends on whether the application is local (run on the same computer and by the same user as your script) or remote (run on a different machine, or on the same machine but by a different logged-in user). You might also like to know what scriptable applications are included with a default installation of Tiger. (On creation of scriptable applications using applets, AppleScript Studio, and Cocoa, see Chapter 27.)
This book won't teach you how to script any particular application (see "The Scope of This Book" in the Preface). If the application comes with documentation or examples showing how to script it, start with that. For certain applications, there may be third-party books or web pages devoted to the topic of scripting it. The application will in any case have a dictionary (see Chapter 20).