Linux is a completely free reimplementation of the POSIX specification, with SYSV and BSD extensions (which means it looks like Unix, but does not come from the same source code base), which is available in both source code and binary form. Its copyright is owned by Linus Torvalds <torvalds@transmeta.com> and other contributors, and is freely redistributable under the terms of the GNU General Public License (GPL). A copy of the GPL is included with the Linux source; you can also get a copy from ftp://prep.ai.mit.edu/pub/gnu/COPYING
Linux, per se, is only the kernel of the operating system, the part that controls hardware, manages files, separates processes, and so forth. There are several combinations of Linux with sets of utilities and applications to form a complete operating system. Each of these combinations is called a distribution of Linux. The word Linux, though it in its strictest form refers specifically to the kernel, is also widely and correctly to refer to an entire operating system built around the Linux kernel. For a list and brief discription of various distributions, see http://sunsite.unc.edu/LDP/HOWTO/Distribution-HOWTO.html None of these distributions is ``the official Linux''.
Linux is not public domain, nor is it `shareware'. It is `free' software, commonly called freeware or Open Source Software[tm] (see http://www.opensource.org), and you may give away or sell copies, but you must include the source code or make it available in the same way as any binaries you give or sell. If you distribute any modifications, you are legally bound to distribute the source for those modifications. See the GNU General Public License for details.
Linux is still free as of version 2.0, and will continue to be free. Because of the nature of the GPL to which Linux is subject, it would be illegal for it to be made not free. Note carefully: the `free' part involves access to the source code rather than money; it is perfectly legal to charge money for distributing Linux, so long as you also distribute the source code. This is a generalization; if you want the fine points, read the GPL.
Linux runs on 386/486/Pentium machines with ISA, EISA, PCI and VLB busses. MCA (IBM's proprietary bus) is not well-supported in 2.0.x and earlier versions, but support has been added to the current development tree, 2.1.x. If you are interested, see http://glycerine.itsmm.uni.edu/mca
There is a port to multiple Motorola 680x0 platforms (currently running on some Amigas, Ataris, and VME machines), which now works quite well. It requires a 68020 with an MMU, a 68030, 68040, or a 68060, and also requires an FPU. Networking and X now work. See news:comp.os.linux.m68k
Linux runs well on DEC's Alpha CPU, currently supporting the "Jensen", "NoName", "Cabriolet", "Universal Desktop Box" (better known as the Multia), and many other platforms. For more information, see http://www.azstarnet.com/~axplinux/FAQ.html
Linux runs well on Sun SPARCs; most sun4c, sun4m, and sun4u machines now run Linux, with support for sun4 in development. Red Hat Linux is (as of this writing) the only Linux distribution available for SPARCs; see http://www.redhat.com/support/docs/rhl-sparc/
Linux is being actively ported to the PowerPC architecture, including PowerMac (Nubus and PCI), Motorola, IBM, and Be machines. See http://www.cs.nmt.edu/~linuxppc/ and http://www.linuxppc.org/
Ports to other machines, including MIPS (see http://linus.linux.sgi.com and http://lena.fnet.fr/) and ARM, are under way and showing various amounts of progress. Don't hold your breath, but if you are interested and able to contribute, you may well find other developers who wish to work with you.
Linux is no longer considered to be in beta testing, as version 1.0 was released on March 14, 1994. There are still bugs in the system, and new bugs will creep up and be fixed as time goes on. Because Linux follows the ``open development model'', all new versions will be released to the public, whether or not they are considered ``production quality''. However, in order to help people tell whether they are getting a stable version or not, the following scheme has been implemented: Versions n.x.y, where x is an even number, are stable versions, and only bug fixes will be applied as y is incremented. So from version 1.2.2 to 1.2.3, there were only bug fixes, and no new features. Versions n.x.y, where x is an odd number, are beta-quality releases for developers only, and may be unstable and may crash, and are having new features added to them all the time. >From time to time, as the currect development kernel stabilizes, it will be frozen as the new ``stable'' kernel, and development will continue on a new development version of the kernel. Note that most releases of the Linux kernel, beta or not, are relatively robust; ``stable'' in this context means ``slow to change'' in addition to ``robust''.
The current stable version is 2.0.35 (this will continue to change as new device drivers get added and bugs fixed), and development has also started on the experimental 2.1.x kernels. The Linux kernel source code contains a file, Documentation/Changes, which explains changes that you should be aware of when upgrading from one kernel version to another. However, the great majority of Linux users simply update their Linux distribution occasionally to get a new kernel version.
Most versions of Linux, beta or not, are quite robust, and you can keep using those if they do what you need and you don't want to be on the bleeding edge. One site had a computer running version 0.97p1 (dating from the summer of 1992) for over 136 days without an error or crash. (It would have been longer if the backhoe operator hadn't mistaken a main power transformer for a dumpster...) Others have posted uptimes in excess of a year. One site still had a computer running Linux 0.99p15s over 600 days at last report.
One thing to be aware of is that Linux is developed using an open and distributed model, instead of a closed and centralized model like much other software. This means that the current development version is always public (with up to a week or two of delay) so that anybody can use it. The result is that whenever a version with new functionality is released, it almost always contains bugs, but it also results in a very rapid development so that the bugs are found and corrected quickly, often in hours, as many people work to fix them.
In contrast, the closed and centralized model means that there is only one person or team working on the project, and they only release software that they think is working well. Often this leads to long intervals between releases, long waiting for bug fixes, and slower development. The latest release of such software to the public is sometimes of higher quality, but the development speed is generally much slower.
For a discussion of these two models, read ``The Cathedral and the Bazaar'' at http://sagan.earthspace.net/~esr/writings/cathedral-bazaar/ by Eric Raymond.
As of September 1, 1998, the current stable version of Linux is 2.0.35, and the latest development version is 2.1.119.