Linux
Mac OS X is based on BSD, which is similar to
Linux. Because of this, it is fairly easy to port programs between the two operating systems. However, the two operating systems are not the same, and programs for one don't work on the other.
Linux also uses the X window system, and Mac doesn't, though Mac has a program to support programs that use the X window system. Because of this,
Linux programs can be configured to run on Mac OS X with a little elbow grease, but not the other way around.
