Secure boot is one feature of the Unified Extensible Firmware Interface, or UEFI, the replacement for the BIOS on common hardware devices like PCs and laptops. (This is commonly referred to as the UEFI BIOS and I shall use this term right through this article).
Microsoft has implemented secure boot in a certain manner and since all hardware manufacturers will be following suit, any vendor of an operating system other than Windows has to follow Microsoft's system to get their operating systems to boot on machines on which secure boot is in operation. All devices that come with Windows 8 pre-installed have secure boot turned on.
PC motherboards that sell in retail come with secure boot off. You can turn it on after building your machine. The boards available in Australia at the moment need a UEFI BIOS upgrade before they will support secure boot.
Microsoft controls the key signing authority and has outsourced the job to Verisign; thus any keys purchased have to be bought from Verisign. Each key costs $US99.
On the x86 platform, Microsoft has mandated that there needs to be a way for secure boot to be turned off. This is not due to a sudden bout of kindness; if Microsoft did not do so, then there would probably be another anti-trust lawsuit given the dominance that Microsoft enjoys on this platform. Vendors are thus required to provide a means of turning off secure boot. However, on the ARM platform, there is no requirement for such a mechanism; there is no fear of an anti-trust suit, at least not yet, as no operating system vendor has a near monopoly of the platform.
Linux (the kernel) has been able to boot on UEFI systems for quite some time. But that does not mean a Linux distribution can boot on a system which is locked down using secure boot. And here is where there is much confusion; one of the most confusing documents I found over the last few days is this, where the reader will come away with the impression that UEFI and secure boot are one and the same thing.
The best way to understand the new technology is to get hands-on and that is what I have done: I have installed Windows 8 without secure boot, and then done an install with secure boot turned on. I built a machine with an MSI Z77A-G41 motherboard for these experiments; given that the layout for the UEFI BIOS differs from one major manufacturer to the next, my conclusions are for this hardware only. Your mileage may vary.