Linux

Tux is the official mascot of Linux

So, if you're really nerds you probably know about this OS, but in case you don't...

Linux is an operating system, like Windows, Mac OS, DOS, etc...
although technically Linux is just the Kernel (the core of the system), the whole system itself should be called GNU/Linux, since the core system tools is from the GNU-project started in 1984, headed by Richard Stallman.

Linux itself started as a hobby project in 1991 by Linus Torvalds, but has since developed in to one of the most robust and beloved OS's out there.

It's quite likely you've already used Linux without even knowing it, since it is the most commonly used system for servers. It is also the core of the Android OS.

Just like Mac OS X, which is based on BSD, is a Unix like system. As such many administrative tools are the same. If you know how to use the text-terminal in Mac OS X, you will probably feel familiar with the Linux terminal as well.

Since everyone is basically free to use Linux for whatever they want, an overwhelming amount of Distributions have been made.
A "distro" is essentially a host of software with Linux at its core, making up a complete OS environment as most people are used to, that enables you to do whatever tasks it is you use your computer for.
The sheer number of distros is often what deters new users from getting started.
But the fact that there are so many, with different focus, means there are probably one available for Everyones needs.

As previously stated Linux is already widely used, but it is not as common as Windows or OS X for use on desktops and laptops.
This is a shame, as I honestly think the correct distribution properly installed is in fact a better OS for most (but not all) people.
It is (especially compared to Windows) faster, easier, more stable, and virtually virus free.

If you won't take my word for it, here's two videos with some arguments why Linux is good.

Personally I have experienced that I become more productive in Linux than Windows, and it's not only because I tend to use Windows for games.
(The notion that you can't play games in Linux is no longer true b.t.w.)

It is possible to have multiple OS's on the same computer, and choose which one to boot in to at startup. This way I can use both Linux and Windows, based on what I'm about to use my computer for.

The biggest reason why more people don't use Linux for their home computers is probably the fact that most PCs comes preinstalled with Windows, and people are lazy. Some computer vendors have even offered Linux preinstalled computers as an alternative, but actually charged more for those computers, despite the fact that Windows costs money and Linux (in most cases) is free!

To get Linux, simply download the installation image from the homepage of the distro you are interested in, then write it to a disc or usb-stick, reboot your computer, and the installation should begin.

Ubuntu Logo

Obviously the first problem you face is what distro you should use.
If you already have one, you should probably use that. If you know of one you find really interesting you should use that.
Otherwise, if you are not very experienced and just want a system that works "out of the box", you should probably go for Ubuntu, or Mint.
Ubuntu was made explicitly to be easy to use for inexperienced users, their slogan is "Linux for human beings".
Mint was based on Ubuntu, with the goal of being a "modern, elegant and comfortable operating system which is both powerful and easy to use."

If you are more experienced you may want to choose another distro, some popular once that are not too complex to install is Fedora, openSuse, and Debian.

Example of Fedora

Personally I use Arch Linux, but it is a bit more complicated to install, since you have to use the command-line to set everything up yourself. Still, it's not too hard if you have installation guides available, or follow a tutorial.
I realize it is unnecessarily complicated for most normal users though, and at this point the argument that Linux is easier than Windows would no longer hold true.

When I started using Linux myself, I used Ubuntu, but since everything just worked "out of the box", I didn't really have any reason to learn terminal-commands, or how the Linux file-system works and such.
Since I actually wanted to Learn Linux, rather than just use it (as is probably the case for most normal people) I experimented with other distros, starting with Debian, as it uses the same package manager as Ubuntu (Ubuntu was originally a fork of Debian).
Later, by installing Arch, I forced myself to learn things I otherwise wouldn't have learned, like how to use text-tools to partition the hard drive, and create and mount file systems, etc.

Example of Arch Linux

Hopefully I have now sparked an interest in this amazing OS, and you might even try experimenting a bit with it yourself.

I might add posts on how to install specific distros, but I probably won't bother with Ubuntu and Mint, since it should be rather easy to understand. Unless someone actually wants me to? In that case tell me in the comments.