You can run the following types of applications under Mac OS X:
Classic applications Classic applications are those designed to run under previous versions of the Mac OS; however, because of the Mac OS X's Classic environment, you can also run these applications under Mac OS X.
Unix applications Because Mac OS X is based on Unix, you can run many Unix applications on your Mac. Some of these applications have to be recompiled to run on the Mac, but most will work as they are. Of course, you will need to run them from the command line (unless you find and install a graphical user interface for the Unix subsystem). Because Unix is such a prevalent OS, there are thousands of Unix applications you can use.
Java applications You can run applications written in the Java and Java 2 programming language. Because Java is a platform-independent programming language, the same applications will work on Windows, Macintosh, and other platforms. You mostly encounter Java applications on the Web, but you will also find some standalone Java applications as well.
Carbon applications These applications are written using the Carbon programming environment, which is designed to take advantage of the Mac OS X architecture. Many are Classic applications that have been ported over to Mac OS X?in Mac OS X-lingo, they have been carbonized. Because carbonizing an application requires considerably fewer resources than does creating a Cocoa application, most Mac OS X applications were carbonized Mac OS 9 applications early in Mac OS X's life. As Mac OS X continues to mature, this will change, and the majority of applications will be written specifically for Mac OS X (using Carbon or Cocoa).
Cocoa These applications are written using the Cocoa programming architecture, which means that they take full advantage of all the advanced features that Mac OS X provides. Cocoa applications have to be written from the ground up in the new environment rather than being ported over as carbonized applications can be. Eventually, most new Mac OS X applications will be based on Cocoa.
To learn about installing and using Classic applications, see Chapter 7, "Working with Mac OS 9, the Classic Environment, and Classic Applications," p. 163.
As with other major transitions that the Mac OS has made, such as from version 7 to version 8, it will take some time before all the Mac applications available are fully up to date with the new OS (those being written using the Carbon or Cocoa architecture). Fortunately, because it is relatively easy to carbonize an application, many developers released carbonized versions of their applications shortly after Mac OS X was released. And for those that haven't been updated for Mac OS X, there is always the Classic environment.
Because many carbonized applications are applications that were created for earlier versions of the OS and ported over to Mac OS X, some do not meet some standard Mac OS X interface conventions. For example, some of the Mac OS X standard menus you see in Cocoa applications are not present in carbonized applications.