What is the difference between an ivus and a cardiac catheterization?

Part of same. Ivus (intravascular ultrasound) is a diagnostic tool that can be used during cardiac catheterization when the angiographic pictures are not sufficient to determine the severity of a blockage. Ivus can be considered part of cardiac catheterization in a subset of patients who require it.
IVUS . A heart cath is the procedure to place a catheter in the heart arteries. Usually contrast is injected to see blockages. Ivus is intravascular ultrasound which may be performed during the heart cath to clarify questionable blockages.