C-tard is a term invented and popularized in England. The full term is ‘computerely retarded’ (sic) but was later shortened to C-tard. It is a slang term to describe someone who has little experience or is just plain stupid when it comes to computers. The term can be used in a derogatory way but is often used for its comedic value.
Jeff: What the hell is this 'windows' I keep hearing about
Cloud: Damn C-tard