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 OS, originally written (coded) in assembler in 1969 by Dennis Ritchie and Ken Thompson at Bell Labs. Later Unix was written (coded) again in C (its native language) for portability in 1972. Unix is the most portable and reliable OS.
By the way, UNIX© is not the same as Unix. UNIX© (all capital letters and often presented with the copyright symbol) is a trademark while Unix (a proper name) is a philosophy. Considering the latter, BSD and Linux are not UNIX©. They are UNIX© clones, but nonetheless they are considered Unix. If you are confused, you are not alone.
At the same time, there are several Unix clones like BSD and Linux. For those who work with Windows and do not want to handle a separate OS installation, Cygwin is handy with its Unix-like (Linux-like) environment.
About 90% of the internet relies on a variation of Unix (normally either Linux or BSD), with Apache Software Foundation (ASF). For example, this web site runs on Linux, with Apache, SSI, Perl, PHP, MySQL and Sendmail with FormMail. If you are new to these technologies and would like to try them, go to Apache Friends where you can download everything in one package.
Most (if not all) source code from the various Linux distributions and BSD, Apache, PHP, MySQL and Mozilla are open source. Support the open source community. One way you can do this is by supporting organizations like the Open Source Education Foundation.