The original UNIX from Bell Labs was provided free on a tape. Later on the OS diverged into different directions, like BSD or System V. During the late 80's and early 90's the OS releases became very proprietary - Sun had one, Hewlett Packard another, and so on. This proliferation of UNIX systems also is known as flavors.
Things are opening up in part because of the open source movement. So, everyone realized that having different commands (or C functions) for the same thing or commands with the same name with different behaviors was a bad thing. So we have an alphabet soup of proposed standards.
Currently, POSIX standards are the one most OS developers try to follow. This means that development is easier when you have to write the same code for Solaris, HPUX, and Linux.
The reason we have to know what OS we are talking about is that some have "bizarre"
features. If we give an answer without knowing which flavor, we may give a bad answer. The Solaris version of awk is an example of a kind of bizarre version of a UNIX tool, for example.
Maybe try downloading knoppix, it is smaller than some others. Damn Small Linux is smallest UNIX meant for desktops ~50MB.
DSL information
The download time is a function of your connection speed. If you are at dialup speeds something like knoppix will take overnight. 1.5Mb DSL connection maybe several hours.
