As discussed on the Linux vs Windows page, distros provide an easy way of installing and maintaining a reasonably up-to-date Linux-based system.
There are large numbers of distros (see distrowatch for more info) which vary widely in what software they include - in fact, the only thing they all have in common is that they include a Linux kernel. But they fall into several broad categories:
- the most analagous to MS Windows are the complete systems installed on your hard disk; the installation programs for these generally include partitioning software so that they can be installed in a separate part of the hard disk on systems that already run Windows. This category includes the best-known distros: Red Hat/Fedora, Mandriva, SuSE - all of which are suitable for beginners; also included are the venerable but less beginner-friendly Debian and Slackware. These are full systems, including application software as well as the operating system, which occupy the equivalent of several CDs. The installation process will try and auto-detect your hardware and install the necessary drivers, ask you basic questions like what language you use and what time-zone you are in, and then let you choose what you want to install/use, though for beginners who don’t really know what they want, there’s generally one of more defaults to install, for example, everything an office or home user is likely to want
- similar are complete systems that are not installed but run off a CD-ROM, aka LiveCD (using compression, a CD can contain 2GB of software or more); these can be easily run as a demo or to try out the software without changing anything on the hard disk. The original of these was Knoppix, but there are numerous similar ones. If after trying the software you decide you want to install it and use it for real, most of these include the ability to copy the software to the hard disk.
- a variation of the LiveCD are smaller versions that fit on other storage media such as compact flash or USB sticks, such as Damn Small Linux or Puppy Linux, which fit a full desktop environment with small user apps such as browser, email or word-processor into around 50MB. The smallest, such as Mininux, will fit on one or two floppy disks, but don’t expect a GUI for that :-). Slax manages to fit a complete system including the KDE desktop environment into a 180MB 8cm CD. If you have sufficient RAM, these often include the ability to run in RAM, which is significantly faster than from hard disk, let alone CD. Most of them also include the ability to copy the software to the hard disk too, though Slax does not recommend this due to the special compression techniques it uses. As Flash and USB devices increase in capacity and capability, the boundaries between these systems and LiveCDs are likely to disappear: the latest USB MP3 players, for example, have enough storage to not only carry your favourite songs but also your favourite Linux system as well. And Flash and USB have the big advantage that, unlike a CD-ROM, they can be written to as well as read, so changes can be stored just as easily as on a hard disk.
- specialist systems that set up your PC, for example, as a Linux-powered firewall or webserver
- these are all binary distros: you are supplied with the compiled binary programs which you simply install. The disadvantage of this is similar to that of MS Windows: it’s ok if you like what the distros offer but takes no account of any particular needs you may have. The final type of system is the source-based distro, where you download the source for each of the programs/systems to be installed, and you compile the binaries yourself. There are several of these, but the best known is Gentoo. You state what you want and the necessary component source files are downloaded and compiled. This creates a stream-lined version specific to your requirements, but is time-consuming and again is not for the beginner.
Most distros are run by groups of volunteer enthusiasts with support typically provided via web forums and mailing lists. Particularly within the first and last group, some of them, including some of the best-known, are oriented to commercial organisations, providing support and training for a fee.
Where to get them
Most can be downloaded via the internet (even LiveCD distros can be downloaded as a CD image which can be burned to CD), and most offer some sort of postal service where one or more CDs are mailed for a small fee; as a variation of the latter, many independent enthusiasts/organisations in your local country will mail you a copy for a small fee as well. Also, Linux magazines often include CD or DVD copies.
Generally, if you download, you start off by downloading a small install program which will initialise the system and then download what is needed based on the choices you make.
Updates
Most distros issue new versions on a regular basis 2 or 3 times a year; these releases are issued as a complete package of compiled programs that should all work together properly. Many of them also offer the ability to update your system on an ongoing basis by downloading and installing new versions of individual programs as they are released. Of course, you can install any additional programs yourself independently of the distro, but if you do this, there is no guarantee from the distro that they will be compatible.
Roll your own Linux
You do not have to use a distro, of course; you can create your own system. See Linux from Scratch for instructions on building your own Linux-based system. This is not recommended for beginners, but is a good way of understanding exactly how everything hangs together.
For further info, Wikipedia has a page with comparison tables.