Very little. There is as much variation between the unixes as between linux and unix. This makes linux a useful tool for learning unix if you want to go into a job.
Linux is more modern and aimed at the desktop more, so system administration can be done using very intuitive GUIs, where on most unixes it has to be done from the command line.
Let me know if you want more information, I use various unixes at work (HPUX, Solaris AIX, I even used SCO once a very long time ago) and so can compare.