Welcome to Linux!
Linux is the core of the operational system most commonly used for free software computer environments.
It is the environment in which a very large community of people, with millions of technology 'nerds', study, work, play and interact. Linux users are often interested in understanding and improving it. I'd say Linux is for Windows what Wikipedia is for Brittanica. You may be a simple Linux user just as you can be a Wikipedia reader, but the invitation is always open for you to be a collaborator and co-author of both of them.
What is GNU/Linux?
O GNU/Linux is a set of softwares. The difference is that you can see or edit the source code if you want.
It is not a single software, but lots and lots of them. There are programs for purposes so different as writing books or controlling the computer cooler.
In general, though, we can say GNU/Linux is an alternative to Windows, that is, it is an Operational System you can use on your PC.
It is important to notice that when replacing Windows you will no longer be able to run the programs that rely on Windows, like Microsoft Word and Widows Media Player, for instance. Easy! There is no reason for panic. There are softwares that will play those roles in GNU/Linux, and they are quite popular. You probably have already heard about some of them and are probably using some other on daily basis.
It will certainly not be possible to install .EXE or .MSI files on Linux. You can test Linux without really installing it. This is very useful especially if you are not really sure that all programs you need will run on Linux or if there is no alternative.
Linux Distributions (Distros)
Linux is not a single software. There are so many free software out there that there is something called Linux Distribution, or Distro. A Distro is a selection of free softwares configure to work together nicelly.
There are people and organizations specialized in selecting and organizing such softwares in a coherent package: a Distro.
There are some very important Linux Distributions out there. Here is a very small list:
The softwares are usually chosen based on some criteria. This helps you choose a distribution for you to test. There are distributions focused on office work, movies editing, music composition, information security and so on.
You can find out new distros visiting http://distrowatch.com, a website that maintains an extensive list of linux distros with user comments.
I know nothing about Linux! I have just arrived from Windows World.
If you are just arriving from Windows world there are some things that will sound a little weird at first. One of the main reasons for this is that people use to do different things with Windows and Linux.
If you are beginning on Linux it is very likely that your are either a nerd, a IT student or a professional that has just been hired by a company that uses Linux. In these situations it is very likely that you will use Linux to do stuff that you don't usually do on Windows. That means that the complexity of your tasks will be increased with the complexity of learning a new operational system. If that is your situation, relax. It may sound difficult at first, but you will soon realize Linux is not really more difficult to use than Windows.
On Linux the Desktop is not the same as on Windows. You are probably used to a start button where the programs show up ordered according to the manufacturer, or to a Desktop full of shortcuts to folders and programs. You will probably not find these on Linux.
The first difference to notice is that there are several desktops for you to choose. That means Linux has more than one possible user experience. The most important Linux desktops today are: Gnome, KDE, XFCE and Unity. Each provides a very unique user experience.
DWM is my desktop of choice. It is not particularly beautiful and does not give you plenty of resources either. Surprisingly, these are it's powerful features.