Boot process for ARM-Cortex-A series
The provess varies a lot depending on a particular SoC, but the general sequence is:


Load bootloader (this depends on SoC)
Bootloader initializes the most important parts - crystal, RAM, other stuff (and possibly allows changing the kernel)
Bootloader runs the kernel


The most common open source bootloaders are U-Boot and RedBoot. If you want a hobbyist board to play around with I suggest Beaglebone - that SoC has very good open documentation and the relative U-Boot parts were contributed directly by TI.