As Apple continues to eliminate PowerPC-based CPUs from their computer line up, many folks are asking about compatibility.  Here's an explanation...

The best solution is to use software that has been written with Intel compatibility built-in.  Also known as "Universal Binaries", there are many applications already available that run natively on Intel Mac systems.  Those would include most of Apple's applications, like Mail, Address Book, iTunes, iPhoto, Safari, and so on.

Other applications which are not yet updated will run in a sort of emulation mode, using a process called Rosetta.  Rosetta emulates the PowerPC chip, and is built right into the Core of the Tiger operating system.  Speed is very comparable for most applications.  A few specialty apps, which might require intensive mathematical calculations, such as Photoshop when performing renderings, will be noticeably slower under Rosetta.

Keep in mind that all OS 9 applicaitions, previously available under the Classic environment, are 100% incompatible and unsupported on Intel machines.