Somewhat similar to the concept of a frenemy, a "relatard" is someone who is retarded (not in the literal sense) that you only associate with because they're related to you by blood or marriage.
A regtard is a Counter-Strike (or any FPS game for that matter) player that plays regularly on a public server and generally makes an ass out of themselves the entire time.
This type of person is prone to typing things like "OH NOES I'M DEAD!!!" and "OMFG soandso's a h4x0r!!!".
More often than not they are under the age of 17 and live in Canada.
A Regtard is an offensive slang term that it used against rednecks or hicks. It is a combination of the term retard, and the popular stereotypical redneck name, reggie. Reg-Tard.