The term, "Truther" or "truthers" refers to that group of people who seriously believe that 911 was an inside job -- perpetrated by our government for its own nefarious ends.
At some point, BHO henchmen, who don't care that BHO is not a natural born citizen of the USA and therefore NOT eligible to become president, started calling people who wanted to see his birth certifigate by the nickname, "birthers" --- obviously trying to denigrate them as conspiratorial kooks like the truthers.
Yes, thanks for the correction. My mistake in terminology.