Windows and most proprietary Unix variants have closed source code. This means that only the publisher of the OS has access to the source code. In fact, under the Digital Millenium Copyright Act (DMCA), it is illegal to try to figure out how a copyrighted program (such as an OS) works. As a result, proprietary OSes are designed, developed, tested and maintained by development teams employed by the publisher.
In contrast, Linux is an open sourced operating system. Although it is technically copyrighted and licensed, the license specifically permit free distribution and modification of the source code (and compiled binary). This means that everyone may analyze the source code or use the OS. This also means that anyone can test the OS and report problems. Anyone can debug the OS and fix problems. Anyone can write new code to extend the abilities of the OS.