Roddenberry was asked about it and he said it was Trekkie. He said he invented the thing and he could name what the fans are. I'm sure one of the Trekkies here wll give you the exact quote and where to find it.
With no disrespect intended to anyone here, there is a significant distinction that goes beyond loving fun or being too serious. Trekkers enjoy Star Trek for its entertainment value. They are just as enthusiastic as Trekkies, but in the end, its just entertainment to them. Trekkies adopt Roddenberry's utopian philosophy as a role model for their lives and testify in court in their star fleet uniforms. Trekkers watch the shows and the movies, read the books and even collect some of the toys, but it's just a hobby. Trekkies move to Los Angeles so they can live near the actor they worship.