I just finished reading an excellent article called BSD for Linux Users by Matthew D. Fuller. He gets to the heart of the matter to describe how Linux and BSD are different. Here's an ex cerpt on the idea of the BSD base system:
"The concept of the "base system" is something that, I think, causes the most trouble for people used to the Linux methodology. Which is perfectly understandable, because the whole idea just doesn't even exist in the Linux world.
Linux, from the start, was just a kernel. Without getting into the eternal debate of what an "operating system" precisely consists of, it's easy to state that a kernel by itself isn't very useful. You need all the userland utilities to make it work. Linux has always been a conglomerate; a kernel from here, a ls from there, a ps from this other place, vim, perl, gzip, tar, and a bundle of others.
Linux has never had any sort of separation between what is the "base system" and what is "addon utilities". The entire system is "addon utilities". MySQL is no different from ls from KDE from whois from dc from GnuCash from ... Every bit of the system is just one or another add-on package.
By contrast, BSD has always had a centralized development model. There's always been an entity that's "in charge" of the system. BSD doesn't use GNU ls or GNU libc, it uses BSD's ls and BSD's libc, which are direct descendents of the ls and libc that where in the CSRG-distributed BSD releases. They've never been developed or packaged independently. You can't go "download BSD libc" somewhere, because in the BSD world, libc by itself is meaningless. ls by itself is meaningless. The kernel by itself is meaningless. The system as a whole is one piece, not a bunch of little pieces."
He explains the ports tree:
"The difference between ports and RPM's isn't just that ports compile and RPM's just install. Ports are designed to cover the full range of bits and pieces of installing stuff; encoding and tracking and installing dependencies, packaging, installing and deinstalling, local changes necessary to build on your system, compile-time configuration tweaks... all those things. An RPM is just a binary package. If you want to auto-install dependencies, you have to have a higher-level tool like urpmi or apt-get to do it. And, since it's binary, you have to deal with library versioning conflicts, or missing compile options, or any of the other limitations you incur by not building it on your own system.
And further, ports, like the rest of the BSD systems, are centralized... all those files in that big directory tree are maintained by the FreeBSD project itself. When somebody wrote KDE, for instance, it didn't magically appear in ports trees everywhere. Somebody had to write all the necessary "glue" to build a port for it, then commit the files into the FreeBSD CVS repository so it would be in the ports collection. So again, there's some level of assurance that it works with other things in the ports collection. Any dependencies it has will be there, because it can't declare a dependency on something not in ports."
He also talks about release engineering:
"In a very real sense, BSD systems are constantly developed; I can always update my system to the absolute latest code, irrespective of "releases". In Linux, that doesn't really have as much meaning, because the release process is very different. I think the most appropriate verb for a Linux release is "assembled". A Linux release is assembled from version A.B of this program, plus version C.D of this program, plus version E.F of this program... all together with version X.Y.Z of the Linux kernel. In BSD, however, since the pieces are all developed together, the verb "cut" makes a lot more sense; a release is "cut" at a certain time."
I highly recommend reading this article.