Splines are curves of constantly changing radius. Inventor supports two types of splines: interpolation and control vertex.
An interpolation spline passes through the pick points. The spline is modified with handles on the points.
A control vertex spline passes through the start and end point, but does not pass through the intermediate pick points. The points define a control frame which is used to modify the spline.
After a spline is created, you can change the type of spline in the context menu.