The term "folk guitar" is often used to describe a guitar which is designed for use with nylon strings like a classical guitar, but has a string spacing closer to that of an acoustic guitar and for whatever reason uses a tailpiece with notches for ball-end strings rather than one which is suitable for tying plain-end strings. This latter "feature" has unfortunately resulted in many such instruments being strung with ball-end steel strings and consequently destroyed by the higher string tension.
Using plain-end nylon strings on a folk guitar would require affixing some sort of blobby thing at the end; I would guess that sliding on a bead and then using the flame from a match to melt the nylon string into a blob would probably work, but I've never tried that. Using ball-end folk guitar strings is probably a much easier approach. Tying the knot required for a classical guitar might be awkward with a ball-end string, but cutting off the ball end should yield a plain-end string suitable for use with a classical guitar that would otherwise have trouble with a ball-end string.