In Ubuntu 10.10, we changed the normal amd64 CD images to dual-boot on either BIOS or UEFI systems (UEFI, "Unified Extensible Firmware Interface", is a different kind of firmware found on many newer systems). This was done using a technique known as a "multi-catalog" CD - it contains two boot images, and the specification says that the firmware is supposed to pick the one it can best use.
Unfortunately, even though Macs use a variant of EFI (an earlier version of what's now called UEFI), they apparently can't cope with multi-catalog CDs, and simply refuse to boot them. This left us in rather a quandary: we needed to support UEFI systems, but we didn't want to drop support for Macs either. I therefore created the amd64+mac CD images, which are exactly the same as the amd64 images except that they only support BIOS booting. Macs are happy to boot these in their BIOS emulation mode.