Code in Visual Basic is stored in modules. There are three kinds of modules: form, standard, and class.
Simple applications can consist of just a single form, and all of the code in the application resides in that form module. As your applications get larger and more sophisticated, you add additional forms. Eventually you might find that there is common code you want to execute in several forms. You don't want to duplicate the code in both forms, so you create a separate module containing a procedure that implements the common code. This separate module should be a standard module. Over time, you can build up a library of modules containing shared procedures.
Each standard, class, and form module can contain:
Form modules (.FRM file name extension) are the foundation of most Visual Basic applications. They can contain procedures that handle events, general procedures, and form-level declarations of variables, constants, types, and external procedures. If you were to look at a form module in a text editor, you would also see descriptions of the form and its controls, including their property settings. The code that you write in a form module is specific to the particular application to which the form belongs; it might also reference other forms or objects within that application.
Standard modules (.BAS file name extension) are containers for procedures and declarations commonly accessed by other modules within the application. They can contain global (available to the whole application) or module-level declarations of variables, constants, types, external procedures, and global procedures. The code that you write in a standard module isn't necessarily tied to a particular application; if you're careful not to reference forms or controls by name, a standard module can be reused in many different applications.
Class modules (.CLS file name extension) are the foundation of object-oriented programming in Visual Basic. You can write code in class modules to create new objects. These new objects can include your own customized properties and methods. Actually, forms are just class modules that can have controls placed on them and can display form windows.
For More Information For information about writing code in class modules, see "Programming with Objects."
Note The Professional and Enterprise editions of Visual Basic also include ActiveX Documents, ActiveX Designers, and User Controls. These introduce new types of modules with different file name extensions. From the standpoint of writing code, these modules should be considered the same as form modules.