The sides of an internal and external involute spline are equally-spaced teeth and the curve on the tooth flank is involute. The curves increase the strength by decreasing stress concentrations. There are many different standards in both metric and imperial also including fillet root side fitting, flat root side fitting and major diameter and side fitting.
They can be used to enable one member to slide up and down another member whilst still under driving load. They are also used as a permanent drive when an interference fit is selected.