Before doing any custom programming, you need to understand a little more about how custom programs that you create or download from the Internet can be used in AutoCAD. AutoCAD itself has a variety of custom programs that extend the core functionality of the application. And AutoCAD 2009 also ships with an add-on called Express Tools, which is a collection of custom programs that are not part of the core application. See Book IX, Chapter 4 for more information on Express Tools.
So how do custom programs that you've created or downloaded get loaded into AutoCAD? The approach varies based on the type of custom program. Before you even get a program loaded, however, you need to understand how different custom programs are represented on disk through Windows Explorer.
Custom programs are stored in application files, which can be identified by their file extensions or their designated icons in Windows Explorer. The programming interface in which you choose to write your custom programs determines the application file's extension. At some point, you'll most likely download programs from the Internet or perhaps coworkers will share some custom programs that they've collected over the years. So you'll find yourself working with different types of application files.
Table BC3-1 lists the various application files you might encounter in AutoCAD.
Table 3.1. Application Files
AutoLISP application source ...