What it is
It's an opensource BIOS replacement.
What it does
It initializes the hardware to get ready to boot. Coreboot is located in the BIOS Rom flash and uses NVRAM/CMOS to store different settings and values.
How the boot progress works
The image consists of one or more coreboot parts and one or several Payloads.
What is possible with it
When you're not aware of doing some coding, you can nearly do everything with it what hardware can do.
Clarify some misinformation
Is possible with coreboot, as you can alter nearly all values which are controllable with software on your hardware. Software does in that case mean, software at bios level.
Right now coreboot has no possibility to do easy overclocking on supported mainboards. Overclocking means right now that you need to change some sources and rebuild/reflash your bios. There is no UI/GUI available for such settings or changes.
Look to the Overclocking section. Same suits here, except for PowernNow and maybe some other processor featured underclocking.
support for GUID partition table
As grub2 supports GUID partion table, coreboot does that too as grub2 is availd payload for coreboot. Remember: The payload is located in the BIOS Rom image and even if it's a bootloader like grub2 you don't need to have it on your harddisk when you use it as payload. (That means you don't need to put anything in the protective MBR section of the GUID.)