What are UNIX® and Unix?
"You need Unix-level performance and reliability. You don't have a Unix-level budget."
— Microsoft advertisement (2003)
Unix is one of the oldest operating systems, originally coded in assembler in 1969 by Dennis Ritchie (right) and Ken Thompson (left) at Bell Labs.
In 1972, Unix was coded again in C (its native language) for portability. Unix is the most portable and reliable OS.
By the way, UNIX® is not the same as Unix. UNIX® (in upper case letters and often presented with the copyright symbol) is a trademark while Unix (in proper case, proper name) is a philosophy and an IEEE standard — Portable Operating System Interface (POSIX).
Several vendors have registered versions of UNIX®. They are Tru64 Unix, IBM AIX, UnixWare, IRIX and Sun Solaris.
There are also several clones such Linux. This means that these are not UNIX®. They UNIX® clones, but nonetheless they are considered Unix. If you are confused, you are not alone.
About 90% of the internet relies on a variation of Unix (normally either Linux or BSD), with Apache Software Foundation (ASF) or related technology.
Support the open source community. One way you can do this is by supporting organizations like the Open Source Education Foundation.
If you are interested in learning Unix as well as its clones or other derivatives, you should learn how to use, at least, one shell.