How to boot Beaglebone Black from SD card

Happened before:

Day one: Beaglebone Black is in my hand - now what?

Day two: Beaglebone Black has Angstrom Linux as default OS inside - and you can ssh your BBB!

Day three brought new information as well as new challenges

On day three Beaglebone Black booted from SD card (Debian Linux). I downloaded the latest Debian - the one with text: "Runs on BeagleBone Black as well without flashing the eMMC". After writing the image on an SD card Beaglebone Black was willing to boot. I had to hold the user/boot button down a couple of seconds while booting. When Beaglebone Black was up and running it was easy to ssh and thus take a contact.

Next thing to do is to flash Beaglebone Black eMMC memory. I want to replace Angstrom Linux with something more up-to-date. Maybe Debian Linux or Ubuntu? Another, a bit challenging issue for me is so called u-boot bootloader. Why does Beaglebone Black use such software. How to prepare a proper SD card with u-boot inside?

Edit: on day four I was too busy to study more. However some notifications were made. When SD card was ejected Beaglebone Black booted Angstrom Linux from eMMC memory. I placed SD again and voila! Debian booted from SD with *no need to push user/boot button*! Very convenient!

My BBB was willing to boot from SD card - good!