As more and more people began developing unix, the more they wanted their own implementations. Disagreement among how things should work no doubt lead to small groups branching off to create their own version of unix. Rather than fighting over the design of the operating system, these individuals simply started their own projects. This was made possible because of the idea of open source code.
Many distributions run on proprietary hardware and are targeted at large businesses.
Other free distributions are aimed at the end user who is looking for a powerful and flexible operating system. That in mind, many people broke off previous unix projects to work on their own distributions. Below are many of the flavors found in today's networked world. Keep in mind that the average user will probably be experimenting with some distribution of Linux, BSD, or Solaris. Even among unix flavors, many administrators consider AIX to be quite different than the others.