UNIX is a family of computer operating systems. It is not a single system, but a family that had been developed with common vision of underlying principles and general command set. The systems of UNIX family feature many different flavors: Linux, OS X, Solaris, BSD and so on.
Get "UNIX Essentials and UNIX Core" DVD as to my taste it is the best general course on the subject.
Depending on the choice you will make there are commercial (Solaris, AIX, HPUX, OS X) and free (Linuxes, Free BSD, Open BSD) versions of UNIX are available.