According to the MLA Style Manual✲ , 3d ed., 8.4, f./ff. after a page or line number means “and the following page(s) or line(s)”, but the abbreviation is “no longer recommended”; explicit page numbers are called for, e.g. 25-26. (Note that MLA now also deprecates use of p. and pp.)

Folio (abbreviated fol., but again, the abbreviation is “no longer recommended”) designates either a book published in the bifolium format (each full sheet of paper folded once to produce four pages) or a leaf of a manuscript or book.

MLA does not provide a citation format for unpaginated MSS or books; traditionally, this is done by folio number followed by r (for recto, the front or right-hand page of the leaf) or v (for verso, the back or left-hand page of the leaf), thus: 12v-13r.

EDIT, to ameliorate the provinicialism of the foregoing: ✲ MLA, the Modern Language Association, is the leading professional association of US scholars in critical and historical studies of modern languages, and its Style Manual is followed by most publishers in those fields.