SPLINE allows you to fit the curve of minimum strain 
energy through selected points, or to approximate a helix
curve. A spline entity is a smooth curve passing through a
defined set of nodes or knot points. It has continuity of
slope and curvature at all points. 

There are three environments for creating splines: Helix, 
2-D and 3-D Spline, and Mesh:

Helix

HELIX allows you to create a 3-D cubic spline that is an
approximation of a helix curve of arbitrary orientation.
The direction, pitch, length, axis, and start point and
endpoint of the spline are user-determined.

A helix spline takes on the current color, line type, 
line width, and pen number at the time of creation.

2-D and 3-D Spline:

The 2-D CUBIC, 3-D CUBIC, 2-D CLSD, 3-D CLSD, 2-D AUTO, 
and 3-D AUTO options allow you to define the shape of the
spline by both the location of nodes and user-specified
tangencies at its endpoints. In all cases, a spline is
treated as a single entity.

Mesh

MESH allows you to create a set of lines that approximates
a surface. It allows you to generate a line mesh that
approximates a two-curve, three-curve, or four-curve swept
surface, a tabulated cylinder, a surface of revolution, or
a sphere.
