Some people told me FreeBSD is NOT Unix, is that right? I'm confused. I checked some articles, but the expressions are pretty vague, and I need some clarification.
It all come down to whether you are speaking legally, or from a technology viewpoint. Legally, FreeBSD, like Linux, cannot use the trademarked term Unix. From a technology point of view, FreeBSD is as much Unix as Solaris, HP-UX, or any of the other commercial versions that have paid to be able to be legally called Unix.
Unix is actually a trademark name that refers to System V Unix. So "legally" it is not Unix. But it is related and "looks" like Unix, so it is often called Unix, or Unix-like. http://en.wikipedia.org/wiki/Unix
Legally, something is allowed to call itself "Unix" if it goes through an expensive certification process by X/Open. So no, FreeBSD is not Unix, they wouldn't dream of squandering the little cash they have to go through a multi-year certification that is obsolete the moment they get it (one change, start over). On the other hand, all the BSDs are direct decendents of the original Unix source code from AT&T, hacked beyond recognition by a horde of enthusiasts from all over the world. So yes, FreeBSD is a (genetic) Unix. In the same way, Linux is not Unix, neither legally nor genetically. But Linux was conceived as a faithful workalike of what Unix is all about (it closely follows POSIX), so, yes, Linux is also Unix.
Confused enough? ;-)
As they all say, legally it is not UNIX. Think of it this way; if you had a boat with 100 pieces, every year you would switch one old piece for a new piece and store that old piece in a shed. In 99 years, when you only have one piece of wood from the original boat, it technically still is the same boat, right? But when you remove that last piece of original wood, would it still be the same ship? The BSD's right now are that ship.