Those working with multiple versions of Linux know they are not identical. But the differences are usually more at the level of annoying you rather than preventing you from accomplishing a job.
There are currently nearly 200 distributions of Linux available. The Linux Standards BASE organization is trying to address many of these issues by laying down specifications. In theory, if two distributions both meet the LSB, they will more likely allow application vendors to port to both distributions with the same code. Other areas of difference include available commands and options, how you install and update packages (RPM, DEBIAN), available window manager (KDE, GNOME) and management techniques within subsystems of the operating system -- not to mention drivers, etc.
After spending a fair amount of time searching, I was unable to come up with a chart that would be nice to hang on the wall next to the bag of Cheetos for use as a reminder of differences when jumping between distributions of Linux. An example of great use for those working with multiple non-Linux Unix versions can be found here.
The differences chart on this page compares AIX, FreeBSD, HP-UX, Linux (Red Hat), Solaris and Tru-64. It goes to a nice level of detail in many of the differences, such as how to list kernel parameters, what commands are used to manage logical volumes and so much more. Be sure to check it out if you work with multiple versions of Unix. Hopefully someone will follow the UNIXguide example and make up a chart for various distributions of Linux or just make me aware of the one that probably exists out there somewhere.
Fred Mallett is founder of FAME Computer Education, which provides standup delivery of educational classes on a variety of UNIX and Win32 related subjects.