If you run your nail along roundwound strings, it'll feel like a tightly compressed spring. Flatwounds, should (at least in theory) feel smoother. They're primarily designed for fretless basses, because they're kinder to the fretboard, which is necessary because the strings come into contact with it more on a fretless, thus wear and tear is greater.
I wouldn't say they are primarly designed for fretless basses although I agree that they are kinder to the fingerboard than roundwounds. Actually they the first type of strings to be used on electric bass guitars. They sound a bit more like upright basses than roundwounds with less sustain. They are not as bright and they are generally not ver suiatable for slap. Put them on a P-bass and you're in Jamerson land.