Are you refering to this page in Rhino 5 Help? https://docs.mcneel.com/rhino/5/help/en-us/seealso/sak_curve.htm
In Rhino a line is straight, while a curve can be either curved or straight. So all lines are also curves, but not all curves are lines. “Free form curve” the shape of the curve is not limited to specific types of shapes as straight lines, conical sections (portions of circles, ellipses, parabolas and hyperbolas) and similar.
All lines, polylines, curves and polycurves including conical-sections used by Rhino can be represented by NURBS.
All NURBS curves have knots.
Degree 2 curves are not limited to arcs and parabolas. Degree 2 curves can have arbitrary shapes. It is true that conical-section curves can be represented exactly using degree 2 NURBS curves with special sets of knots and weights. But degree 2 NURBS curves are not limited to those special cases.
Degree 3 and above NURBS curves have knots, just like degree 1 and degree 2 NURBS curves.
Number of knots = number of control points + degree - 1
Number of control points => degree + 1
Except for degree 1 curves there is not a simple association of control points and knots.
Degree 2 curves can have arbitrary shape.