BerryBoo is a bootloader/universal operating system installer. It is a simple boot selection screen, allowing you to put multiple Linux distribution on a single SD card. In addition it allows you to put the operating system files on an external USB hard drive instead of on the SD card itself.
To install: extract the contents of the .zip file to a normal (FAT formatted) SD card, and put it in your Raspberry Pi. Once you start your Pi it will start an installer that reformats the SD card and downloads the operating systems files from the Internet.
This video show booting up Raspberry Pi 2 with BerryBoot, download and install Ubuntu Linaro. Both the default user and password is "linaro".