(Did you mean FEN, the Fannish Educational Network, or a James Kepner apazine?)
Fen is the irregular plural of fan. (Man is to men as fan is to fen. But fans is okay while "mans" isn't. Go figure.) Also neofan → neofen, even (sometimes) femmefan → femmefen.
The terms are used interchangeably, but there's a sense that fen is somewhat more fannish, being unique to fandom, and therefore especially useful when distinguishing between fans, meaning "members of fandom,"and fans, the mundane term meaning "fanatics" or "followers" or "blowers of hot air" (not that we don't have plenty of those in fandom, too).
The plural was popularized after Art Widner, Louis Russell Chauvenet and Norm Stanley, the only three members of Mecon, a fan gathering in summer 1943, solemnly voted its adoption and began using it in FAPA.
|From Fancyclopedia 2, ca. 1959
|Plural of fan, by analogy with man/men; it came into wide use after a Maine conference solemnly voted its adoption. But the term is not universally accepted, and some dislike it.
|From Fancyclopedia 1, ca. 1944
|Alternative plural for "fans", which came into general use after the Mecon solemnly voted its adoption.
|This is a fanspeak page. Please extend it by adding information about when and by whom it was coined, whether it’s still in use, etc.