The most commonly used command for drawing freeform curves in Rhino is curve.

Curve Command

• Curve allows you to define a curve by selecting points sequentially in the Rhino Space.
• Rhino will automatically smooth the curve based on your specified control points.

Degree

• Curve has a subcommand Degree which specifies how closely your curve follows the control points.
  • Degree 1 → Polyline, aka a kinked line
  • Degree 2 → Curve which follows control points closely
  • Degree 3 → Curve which is more smoothed and follows control points less closely.

See extensive documentation of curves here.