A couple of questions about MMC/SD cards and booting the BB

In all documentation about the BeagleBoard the memory card is called
an MMC card. I have never ever to my knowledge seen an MMC card in
real life. Plenty of SD cards though. I checked on Wikipedia and an
MMC card and an SD card are almost the same but not exactly the same.
For use with the BB, are they equivalents?

About a quarter of a century ago we got LBA addressing for disks. Some
of us thought that we will never have to worry about heads, sectors
and cylinders again. Some of us have realized that we were wrong.

Can someone explain to me why the memory card must have the geometry
255/63/xxx (H/S/C) for the BeagleBoard? Or even better, explain to me
that I have misunderstood it all.