Ubuntard is a concatenation of "Ubuntu" and "Retard."
Ubuntu is a piece of software called an operating system. An operating system that most are generally familiar with is Microsoft Windows. Ubuntu is part of the GNU/Linux family of operating systems, and it is designed to be easy to use for those who are not particularly computer-savvy, yet easily customizable for experts.
Retards are people who are developmentally challenged, which means that they may not comprehend everyday happenings and thoughts at the level of what society's normal folk do. They may also act like children by throwing tantrums, thinking they're always right, being hypocritical, etc.
An ubuntard is a special kind of computer user. They are users of the GNU/Linux distribution Ubuntu, however this is not the only defining characteristic. To be called an ubuntard you must generally believe you are an expert in the field of IT, computer science, or the Linux OS in general strictly because you use Ubuntu to browse the web, chat with your buddies, and do other end user things. Ubuntards generally congregate in the Linux/Unix area of the Technology portion of digg.com. You can observe them there in their natural habitat where they normally won't get called out on their 'buntardism by real experts, which in comparison to 'buntards are few in number on digg's tech area.
You may have noticed that you can shorten ubuntard with 'buntard. It sounds like boon-tard, and is far easier to say in conversation.
Man, this fucking ubuntard thinks he is Linus Torvalds cause he uses Firefox on Ubuntu, yet doesn't know what bash is.