According to the American Heritage Dictionary, a colophon is "An inscription placed usually at the end of a book, giving facts pertinent to its publication." As is often the case, sf fans got it backward and so generally put the colophon in the front of their fanzines
. Wherever it's placed, it contains the editor/publishers' names, address(es) (sometimes telephone and/or fax numbers and/or email address(es)), the name of the fanzine, the issue number, copyright notice (if any) and anything else the zine editor may want to put in it.
"The idiot forgot to put a colophon in his fanzine!"