This must have been asked many times, but for some reason it's not clear from about ten mins looking into it
why is there a separate 64bit and amd64?
does this mean 32 bit code won't run on the 64bit, but on;y on amd64?
or simply that intel and amd have diverged and 64 is intel optimized, amd is amd optimised?
amd64 (or x86-64, commonly called 64-bit) is backwards compatible with x86, so software written for the latter can run on the former. The name comes from the fact that AMD came up with it, but Intel adopted it and software for amd64 run just fine on Intel CPUs. Since amd64 extends x86, x86 can't run software written for amd64/x86-64.
Macbooks used to have CPUs of the PowerPC architecture, but they have long since shifted to the amd64/x86-64 architecture. A separate image is offered for Macs not because of optimization, but because of booting issues caused by the UEFI implementation in Macs (see What is different about the Mac ISO image?).
Ubuntu offers three options for the x86 (commonly called 32-bit) and its successor architecture amd64/x86-64 :
x86 imageamd64 image, UEFI/BIOS bootable.amd64 image, BIOS bootable (i.e., UEFI systems will boot them in the Legacy boot mode).
