There are two major Unix types: at a certain time the kernel code was forked and two development paths were formed. The one followed at Bell Labs AT&T is called System V (five) Unix, and there's the Unix developed at Berkely University called BSD Unix. Unix vendors today consist of a mixture of these two systems, providing compliance with the POSIX standard, which is a best of breeds unifiing the two systems again into one standard, although POSIX is mostly based on System V Unix.
Originally developed as a hobby project by Linus Torvalds, a student who wanted a better system than Minix that would be suitable for academic use (meaning freely available and useable also by people that don't have a mainframe in their living room), Linux is now a mature operating system providing all features of other Unix implementations, and many more that aren't found elsewere. Today Linux is a complete Unix clone, capable of operating workstations as well as middle range and high end servers. The major vendors all have developers contributing to the further developement of Linux.
Although there are a large number of Unix implementations, you will find a lot of similarities in the different dialects, if only because every Unix machine is a box with building blocks that you may put together following your own needs and views. When you get the system, that's only the beginning of a longterm relationship: you start building it, but it is never finished. Just when you think you have a nice running system, it will stimulate your imagination and creativeness, and the more you realize what power a Unix system can give you, the more you will try to redefine its limits.
Of course there are minor differences between Unices (e.g. the same operating system will look different on a handheld device than on an accounting server in a banking company), but the basis is always the same. This guide will try to be as general as possible so that it can be useful to any beginning user, what Unix flavour he or she may have chosen. Refer to your system documentation for system specific details.