t's an Unix-like operating system. You can get it on the web. Just select a distro you like, download the ISO-file, and burn it to a CD or DVD. Personally I mainly use Linux... it can be month between each time I boot into Windows (you can set-up your system to have both).
Firefox and Opera -- as well as some Linux-only browsers -- are available, so it should be little difference surfing the web. There are several messenger-clients, some -- like kopete -- can handle several protocols (ICQ, MSN, Y!M, AIM, Jabber), so you can use the same client for all your accounts. There are also several IRC-clients available. There are several good text-processors, spreadsheets and/or office-suites available; but OpenOffice is probably the best.
You can't run CorelDraw and Adobe Master Collection directly (they're Windows programs), but there are some fairly good alternatives -- InkScape (vector-based) and GIMP (bitmap-based). You may be able to run CorelDraw and Adobe under Linux using Wine (a Windows-emulator) or a actualization-program like VirtualBox (it creates a "virtual PC" where you can install Windows -- or other operating-systems).
As a Linux-novice, I think one of the Ubuntu-distros is most suited. Ubuntu uses a desktop-enviroment called GNOME... kubuntu uses one called KDE, which is the one I prefer... and xbuntu uses a more lightweight enviroment called Xfce. Personally I think KDE is better stitched together than GNOME. In any case, you can always install (one of) the other desktop-enviroments later.
The main difference between the distros, is the format of the software-packages. Programs packed for one distro, will probably not work for another -- partly because of binary and/or dependency (other packages a package need to work) incompatibility, and partly because how the distro adds software (the format varies). However, getting the source-code and building the package yourself, is almost always an option. Then they differs a bit on how easy they are to install and how easy they are to set-up. There may be a difference in available packages and what packages are installed as standard, but you can usually download and build what you may be missing yourself. Finally they may use different desktop-environments/WindowsManagers -- usually KDE or GNOME though.
One of the great things about Linux, is that it comes with all sorts of servers that can be installed. You can set-up a secure-shell server, so you can log-in at home from work... you can set-up vnc-server, so you can run X-programs (X is the GUI under Linux... think Windows) on your computer and view the elsewhere... you can set-up a POP or SMTP server to handle mail... you can set-up an FTP-server...
...Or you can set-up a web-server, with PHP -- and perhaps message-boards and photo-galleries. Admittedly, due to the way most home-computers are connected, a home-PC doesn't make a very good web-server for others to access... But as you can run your own server and install your own PHP-packages, it's a great help when developing and testing a web-site -- before you publish it on your web-hotell.
There are tons of programs you can download from the distro's repository and install.
A few finishing words:
Linux is based in the Unix tradition, and under Unix there where a few experts -- administrators -- that kept everything running smoothly, while the rest of the users just "used" the system... on a home-PC, you have to assume both these roles. Although most administration has become pretty easy, there are still some rather obscure files you may have to edit by hand if you're unlucky. So it's a bit steeper learning-curve than with Windows. The best is to start with default packages, and be a bit careful when adding more -- especially servers (which may pose a security-hole if they're misconfigured). Only use administrator-privileges (becoming the root-user) when you absolutely need to... do as much as possible as a normal user.
Unix is a multi-user OS, so it's very easy to have many users sharing a computer. Traditionally, most user logged-in to Unix-systems through terminals or over the network... only the privileged few worked "at the console" -- on the terminal in the server-room.
In Linxu, the Graphical User Interface -- X -- is just a program running on top of the OS. It's not an integral part of the OS. It's a bit like old MS-DOS and Windows 3.0 (but unlike MS-DOS, Linux have no problem handling many running programs at once). X only provides the bare minimum for drawing windows, so on top of X, comes a WindowManager -- and perhaps a DesktopManager -- that lets drag around windows and resize them and such. As the WindowManager/DesktopManager is a separate program, and neither integrated to X nor the OS itself, you can freely choose the WM/DM you feel most comfortable with. Linux itself, is text-based (like MS-DOS); and many things can be done quicker and easier with the command-line than the GUI. However, most Linux-distros today start X at boot-time.
The Unix-philosophy is that a command/program does *one* thing, but does it well. Rather than having a single program that both archives and compress (like WinZip), Linux uses one program for making an archive and another for compressing files (including archives). Rather than using one tool for both searching for files and searching for the *content* of a file; Linux uses one program for finding files and another for looking at the content of a file.
Linux assumes you know what you're doing. It won't hold your hand, and ask if you really want to delete a file -- it will happily delete every file on your system. Linux only complain if it can't do what you asked of it. Of course, you *can* make it ask for confirmation... but that is not the default, but requires an option. This is why you shouldn't do you day-to-day work as the administrator (root), because one day you're not where you think you are as you issued the command to delete everything under this directory. In fact, it's a rite-of-passage for a Unix/Linux admin to trash their first system this way... though extra credit is given to those who had taken back-up.