District F is Founders District. It is the only district with a letter instead of a number. It is called Founders District because the Founder of Toastmasters, Ralph Smedley, started the first club in Santa Ana, California, within the geographical confines of what is today Founders District.
The first club existed before the first district: District F didn't exist until 1944. Since then, there have been a long list of district leaders, with the top title currently known as District Director. World Headquarters is, as of 2016, located within District F.