This website is THE reference for the Linux kernel sources.
You'll find there all the sources of successive official kernels in different formats.
You'll also find documentations, utility software, and other projects related to the Linux
kernel. (Compiler, libraries, file systems, ...)
I finally found a way to explain what a distribution is to anybody not accustommed to Linux (or
rather, to GNU/Linux or even UNIX).
A distribution for Linux is like a cook for your meal. I do not restrain the example to
restaurants as, like it's culinary equivalent, you can take on your apron and sharpen your
knife and start building your very own linux installation. (That's called LFS : Linux From
Scratch. Here's the
cook book.)
For your meal, you've got many options. If you don't want (or have time) to start cooking
your meal yourself, you can go to the fast-foot, to the cefeteria, or to the restaurant. This
choice will settle what you'll get in your plate (on your computer).
Let's suppose you've already choosed what you want as a meal for today (set up your project)
and all the fast-foods, rastaurants, and yourself, have the same choice of markets and stores.
The products are the same.
Some of the restaurants will not have what you want for today (the fast-foods for example,
equivalents to the Live-CDs -- though this is not necessarilly true and quite reductive).
Then some cooks have a mind at choosing better food, fresh vegetables, ripe fruits, tender
beef. (Let's say the latest version of this or that software, more stable, or more user
friendly).
Some cooks will create sauces which will not be to your taste, or no sauce at all. Others
will provide unmatched dishes or will let you choose on your own.
Then, the service and layout are an important part of your meal. All are not equivalent.
The Debian GNU/Linux distribution is my favorite.
Let's imagine a restaurant in which you have all freedom, where the cook does no mistake (if
he does, you can fix them !), where the waiter will always tell you which wine the best for
what you choosed to eat, where you'll find nearly any product to be sold, knowing where it
comes from, being able to pick on your own or to follow the guidance of a menu of your choice,
being able to taste aventurously the latest discovery from the kitchen or rather some
well tested recipe, even being able to restrain the choices if you are of a given faith or
vegetarian.
You'll have the debian GNU/Linux distribution.
Nothing more than what you'll be able to find on your own, but being able to choose with
the guidance of the experts.
Of course, as for the cooking, all boils down to your very own tastes.