The Wave Generator is an assembly of a thin-raced ball bearing fitted onto the periphery of an elliptical cam. The inner ring of the bearing is fixed around the cam causing the outer ring of the bearing to conform to the same elliptical shape of the assembly. The Wave Generator is usually attached to the input shaft.
The Flexspline is a thin-walled steel cup with gear teeth machined into the outer surface near the open end of the cup. The bottom of the Flexspline (cup bottom) is called the diaphragm. The diaphragm is usually fitted to the output shaft.
The Circular Spline is a rigid steel ring with teeth on the inside diameter. The Circular Spline has two teeth more than the Flexspline. The Circular Spline is usually fixed to a casing.