The process of installing OS X goes though 3 stages: preboot, install, and postboot.

In order to run, os x needs:

An HFS+ partition (preferably on a GPT partitioned disk)
a bootloader
some drivers and/or patches


The Preboot

By using a special bootloader, usually on cdrom or usb-stick preboot media, you load some drivers and patches into RAM, providing a bootable environment for your OS X media.


Most usually the preboot media will contain:
An Extensions folder with a number of kexts, or an Extensions.mkext file
and sometimes also:
Kernels like mach_kernel.voodoo
Configuration files like com.apple.Boot.plist
Patches such as DSDT.aml

In order to boot the preboot environment needs to provide certain things:

A decryptor is the absolute minimum. This is sometimes compiled into a patched kernel, most commonly provided as a kext, and occasionally even built into the bootloader itself.

A disabler is often useful, This is aso sometimes found in a patched kernel, but most commonly this is a specialised kext. Its purpose is prevent certain kexts from loading, this is sometimes needed to avoid the loading of kexts that cause kernel panics in particular machines.

Injectors inject certain values into the system most commonly to enable hardware to be recognised. OS X expects information from EFI at boot. On a BIOS machine, this information must be provided from elsewhere, and this is done through numerous ways, from the bootloader itself, through "device property" values and DSDT patches to specialised kexts.

Drivers
if you have hardware that isn't supported by the vanilla kexts in OS X, then you may want to have the drivers included in the preboot.

Types of preboot media:

The most common form of preboot media is what has become known as the "Boot-132" CD This is a cdrom booting with isolinux or grub as a bootloader, launching the boot-132 bootloader or one of its derivatives, which then loads a disk image containing the decryptor, disabler, plus any injectors or drivers, in the form of kexts into ram and then asks what to boot.
Numerous variations on these CD's now exist that can include all the types of patches currently available.

More recently there is Chameleon cdboot, which allows you to go directly to the chamelon bootloader without using isolinux or grub, which simplifies the process.

USB sticks have many and varied uses, amongst them preboot. it is also possible to clone your retail DVD to a usb stick itself for install.

The Installation:

Once the preboot is complete, the machine is ready to boot the OS X media. this is most commonly the retail OS X DVD. with a cd preboot, you will usually swap the cd for the DVD and boot that by hitting a key.

the os x install media will boot, and you will be presented forst with a language chioce, and then with (can't bloody remember the sequence of screens) Customise if you like, use disk util to format GPT & HFS+, run install at the end, if ot says failed cant make bootable, ignore.(will expand later)

After installation, comes the first boot. For this you proceed exactly as for the installation preboot, but select the installed
harddrive instead.

(1st boot sequence)

once sucessfully booted, the tasks are:

Installing a bootloader: this allows you to boot the system without the preboot media
Finding out what works and doesn't: sometimes you need to adjust BIOS settings or change hardware around to get things just right

Installing drivers, injectors, kernels, patches as appropriate.

at the end of this process you should be able to boot into a system working as well as its hardware will allow.

THIS IS A WORK IN PROGRESS...