Radius is the radius of the circle that could be made following the curvature of the fretboard/frets. A larger number for the radius makes for a flatter fretboard.
Scale length is the distance from where the strings break over nut to where they meet the saddles.
I'd add to what Pete says by saying that the scale length of the guitar is one of the factors that determines the string tension on your guitar.
If string gauge and the pitch the strings are tuned to remain constant then typically a Gibson type guitar (24.75" scale length) will typically feel 'slacker' or 'looser' to play than a Fender (25.5" scale length) guitar.
Having said that there are other factors that affect how 'taut' a guitars strings feel.
Darin, maybe this helps. The JP is flatter across the fretboard, and is also skinner underneath the fretboard, then the Silo. The Silo has a more pronounced C shape, and the fretboard has more of a curve.