As we are talking about applications in OS X, and if you are discussing you Mac with other Mac users or checking out web sites, you may hear the terms cocoa, carbon, and classic thrown around. Never fear, here's what those crazy words mean.
Cocoa
A cocoa program is one that is written from scratch for OS X. The software company has used the OS X specifications to create the program specifically for the operating system. Cocoa programs generally arrive on the scene after OS X's release in brand-new programs.
Carbon
A carbon program is actually an OS 9 program that has been updated for OS X. In other words, the software company didn't throw out all of the old OS 9 code but simply updated it for OS X. Carbonized programs don't have access to all of OS X's functions as the program would if it were written as a cocoa program, but carbonization saves software companies a lot of time and money is program development. In fact, many of the programs that came bundled with OS X are actually carbonized programs, including iPhoto, Internet Explorer, and even the Finder.
Classic
Classic programs have not been updated for OS X at all. They contain the same programming that was used under OS 9. Some software companies simply do not update their existing software, or you may want to use a program from a company that is no longer in business and cannot update the software anyway. That's no problem; OS X can still run these programs for you through a simulator called Classic, although this mode is less stable than the OS X environment.