Asking for the best distribution is a bit like asking "what is the best vehicle?". I assume you mean best desktop distro though.
For newcomers it is generally suggested you steer clear of the more exotic/specialised distributions unless you actually need something it does (even then you can usually download the package to turn one of the distros into what you need- if playing home user most likely XBMC
http://wiki.xbmc.org/index.php?title=Insta..._XBMC_for_Linux which can turn an old computer into a top flight media box)
I should also mention liveCDs and virtual machines.
LiveCDs (although you can twist a USB drive into them these days) boot the machine into them bypassing the OS on the hard drive
Virtual machines are emulators by any other name and can emulate a full computer in which you can run whatever you like. You can also flip it around and run a virtual machine in linux if you need to run something that only works in windows and a compatibility layer like WINE is not cutting it.
LiveCDs are great when you have a virus and need to pull things off a machine but aside from that they can be used to make sure your hardware plays well with linux (things are pretty good these days)
Nimbus covered most of what I suggest for a first spin although I might add puppy linux which is designed for use as a liveCD and can be made very lightweight (it pretty much is to begin with).
Personally if I had no other distro in my pouch of discs it would be puppy linux but that is because I find it so useful for general messing around and what I tend to do (grab things from hard drives and couple it with a USB drive to allow me to have "my" computer wherever I happen to be) although if I do another for more general use/showing off it would have to be linux mint closely followed by arch and opensuse.
I quite like debian for servers of various flavours and although it gave rise to ubuntu (which gave rise to mint) I am not convinced it holds for a home PC, if doing corporate computers I am quite happy to have debian though.
In no real order
Ubuntu, Mint, Debian, OpenSuSE, Arch Linux and puppy linux.
If you have the time download a copy of each liveCD and burn a couple of discs or run it in a VM to get the hang of things.
This is not getting anywhere really though so a quick overview of linux.
linux as mentioned in a previous post is a kernel. This runs the machine more or less.
On top of this is usually stacked X11 (this provides a kind of command line and a few choice things at a slightly higher level). Some of the network managed distributions stop here (or sometimes before).
On top of this is the window manager and/or desktop environment- GNOME and KDE are the "big two" and come with a lot but XFCE, unity and LXDE are others that might be quicker and have less stuff bundled.
On top of this you can have the file managers and things like compiz which do the 3d effects you might have seen
You can pick what you like in the way of window managers. It makes little difference and for the higher end ones you can install any underlying libraries should a program need it (you can run a "KDE program" on Gnome as long as you have the KDE libraries installed and vice versa and so forth).
Of course all these are customisable at various levels and the better distributions wind it all together and customise them according to whatever they feel like. Distributions also bundle programs (some of which are low level programs- the command line might change between distributions although it is usually fairly similar) although it tends to mean web browser, office program, image viewer, some computer management software but this brings us to the next topic of packages.
On windows one tends to download a file named something like setup.exe and run it leaving the setup to sort everything and do what it likes. Linux considers this a stupid idea (and from a security standpoint it pretty much is) so you have packages and repositories aka repos.
Each distribution (although there is a measure of cross compatibility for some) has a method of installing software which is why you sometimes get a long list of downloads for effectively the same file (
http://www.winehq.org/download/ is a nice example). As the end user you find it, press run, allow it to run if the OS asks you and watch it install. There are advantages and disadvantages to different methods but ignore that debate.
Alongside this though you have something you might not be used to if you have windows. Repos are like mini websites (your distro will tend to have a management program for this). They tend to be slightly behind the websites but not enough that you have to worry.
Depending on your distribution you might have to add a "third party" distro to add things like DVD playback and audio codecs (patents getting in the way of things or them doing things the main distribution might not want to be directly associated with).
Of course you often also have the ability to download source code and turn it into a working program but that can be left for later.
