Mac OS X applications that can run natively on both Intel processor- and PowerPC processor-based Macs are called Universal applications. Even if you don’t have a Universal version of an application, you can still use it on an Intel processor-based Mac by means of Rosetta, which comes with every Intel-based Mac. Rosetta works behind the scenes to translate an existing, native, non-Universal application (one that was designed to run natively a PowerPC-based Mac, not a Classic application) so that it can run on an Intel-based Mac—all you have to do is double-click the application!
Sometimes, you might have a Universal version of a graphics or Internet application on your Intel processor-based Mac that you want to run via Rosetta instead of running natively. Because some Universal applications may continue to rely on existing plug-ins, you may sometimes need to force a Universal application to run in Rosetta to accommodate the plug-in. Check with your plug-in manufacturer(s) to see if Universal versions are available, too.