When Apple switched from PowerPC to Intel processors in 2006, it was important that programs written for PowerPC processors would still run on the new Intel-based Macs. Therefore, Mac OS X for Intel computers included Rosetta, a technology that enables PowerPC programs to run on Intel processors.
Rosetta dynamically translates PowerPC instructions into Intel commands. Users can't see Rosetta running, but it is active any time a PowerPC program is running on an Intel-based Mac. Since translating instructions from one processor to another takes some time, PowerPC applications may run slower on Intel-based Macs. Fortunately, the slowdown is minimal and may not even be noticed by most users.
You can check if an application uses Rosetta by selecting the application icon and choosing "Get Info" from the File Menu (Command-I). Underneath the "General" section, if the Kind is listed as "Application," it is a PowerPC program, which will require Rosetta to run. If it is listed as "Application (Universal)," the program is a Universal application, meaning it was developed to run natively on both PowerPC and Intel computers. You can also check what currently running programs use Rosetta by opening Activity Monitor in the /Applications/Utilities folder. Underneath the "Kind" column, each process will be listed as either Intel or PowerPC. Those that are listed as PowerPC are currently using Rosetta to run.