Something like 6 months ago now, a tremor shocked through the osx86 scene with DFE's publication of what might be called the founding principles of the New Way:

http://tgwbd.org/darwin/index.html

Basically he pointed out that we'd been doing it all wrong, and that there was a better, neater, cleaner, more elegant way, which he demonstrated with examples.
A rush of activity ensued, and thanks to the opensource nature of his work, it was quickly picked up on and developed into useful forms. Somehow, however its propagation to the masses has stagnated and here we are, months on, with all the tools available, yet there are still an awful lot of people still using the clumsy old methods.

A quick outline of the difference:

The old way, which was developed painstakingly by trial & error throughout the days of 10.4.* were based on the idea that you take os x and you add, subtract and replace drivers, kernel, bootloader and frameworks on disk until it works for your machine. Due to the fact that the only way to get tiger for intel in the early days was either to buy an intel mac or download it illegally, (the retail DVD being ppc only, and the discs distributed with macs were restore discs, not full installs) pretty soon patches for the developer disks, and later fully pre-patched install DVDs cleverly engineered from developer release DVDs and restore discs became the norm. These were state-of-the-art. and tweaked and patched to boot and install on as wide a range of hardware as possible. Installers for "patches" - drivers modified to work with particular hardware and such were included to make the installation process as smooth as possible. Components were mixed & matched from different versions of os x and Darwin and this patchwork was what people ran. And it worked pretty well, from "almost perfect" to "only just" depending on the hardware people had & how good they were at fixing minor issues.

To sum it up technically, to run tiger you needed: a patched kernel, some means of decryption (initially this was done offline & redistributed as decrypted binaries, later people put the decryption key in the kernel, and finally in a kext of its own) the bootloader and some other bits from Darwin, and drivers etc. Keeping track of this, and finding the magic formula for getting it all on a DVD and working was a job for gurus.

Leopard is an entirely different story. During tiger, clever people had discovered that one really neat way to get hardware to work was to pass the system the same information it would expect to receive from the EFI that was built into the intel macs. This idea took root & pretty soon various modifications to the (opensource) darwin bootloader were made that passed similar information allowing the unmodified kernel to be used provided the hardware was sufficiently close to use it (Intel core-architecture cpu & intel chipset) and as an added bonus there was a DVD you could buy in the store which had all the intel code on it: pirating your copy of os x was no longer an absolute necessity to play.

Lets look at just what a revolution this was: instead of patching, modifying and writing from scratch the drivers and kernel to make various hardware work, the plain apple drivers and kernel could be used simply by passing them the right information. No more patching, modifying, version control, etc etc. clean, neat, and above all separate from the actual driver or kernel itself.
The New Way was about to be born. Why, said the clever people, when os x is capable of patching and modifying stuff held in memory, are we still modifying stuff on disk? and if we can run unmodified os x on our machines with all the patches in ram, why not use the unmodified DVD to install from? And so what is popularly (though incorrectly) called boot-132, the preboot cd , was born.

A summary, then, of new vs old:

Old: Apple os x with modifications on disk to boot & run on pc hardware.

Advantages:

One pirated & hacked disc contains every patch under the sun and the OS as well.


New: Unmodified Apple os x with modifications in RAM to boot & run on pc hardware.

Advantages:

The patches are separated from the OS throughout. if you need to adjust, tweak, replace or add a driver, they are all neatly located in one place, separate from the OS and thus more manageable to deal with.

Updating via software update can be facilitated, because the patches don't get overwritten like they do when they reside in the OS installation itself.

The installation can be done with an unmodified retail DVD and just a small download, no need to download an illegal copy of the OS, No need for endless and confusing options as each setup can be pre-tailored to your hardware.

Common to both is that they can take advantage of the advances in opensource development, notably the voodoo kernel to make AMD *almost* as easy as Intel to install on.

The crucial point here is separation. By having the patches in one place and the os in another, and applying them at boot time via a bootloader, rather than by hand or automated installer, the system can be kept clean and easy to maintain. Checking a box that says "usb fix" on a normal osx86 dvd leaves you with no knowledge of what you installed, where it went, and what it does. If it messes up, you're left with few options but to reinstall. if, however you put an iousbfamily.kext (it's a random example) in your patch folder (usually /Extra/Extensions, but it can be placed in many different places, such as a separate partition, a disc image, or a prelinked kernel) and it messes up, your biggest problem is simply how to get access to that folder to remove it. It's easy to find, it's not in amongst the entire gamut of drivers in /System/Library/Extensions and when removed, the plain "vanilla" item takes over again.