I would go with a history lesson as the first, understanding the origin (AT/T, mega monopolized telephone network company, multics, Bell Labs). Unix was the first operating system programmed in a "high level" language, C (just show a slide of C and assembly and anyone will understand that they are different and one is easier to work with). The C language made it easy for others to understand and help with the source code, which probably shaped the design of the system quite a lot. Same people who created C created Unix. Timesharing. Textbased input via real terminals was the "only" way to "use" it the first 10 years, and the shell is not that much different today, unix philosphy. It is probably good to try to have someone new understand the problems the license caused during the 80's, POSIX, and the relation between "Unix", "Linux" the "GNU" and the BSD's. And for someone "completely new" (venam, it sounds like your want to teach unix to someone who have never seen a computer) or not that tech oriented, more or less all of the stuff above can be explained more as history then tech specs. 40 year old specs can't be that important anyways, right ;)