Spinning off from Ubuntu
Ubuntu is probably the best known desktop GNU/Linux distribution at street level, picking up new users by word of mouth and astute viral marketing. So much so that for many users new to Linux, Ubuntu has become synonymous with Linux. Linux is Ubuntu; and Ubuntu is Linux. But Linux and free software come in many different flavours, and the adventurous user goes in search of wider options, other distributions and new desktops.
The aim of Ubuntu is to appeal to desktop users who have grown up with Windows and the Mac, and who expect a familiar landscape with all the trees and windows in familiar places, who want something that "just works" and don't really care if it's Windows or Linux, as long as it isn't virus ridden or difficult to use. For most users Ubuntu serves its purpose.
What's in a distro?
One challenge with Linux for the majority of computer users is that the shear breadth of choice on offer is extremely confusing. Windows or Mac users at least have a strong unified brand to identify with and for many computer owners the operating system and other software is simply 'what came with the computer'.