Pericardial Effusion and Cardiac Tamponade ,the difference : 

PERICARDIAL EFFUSION: Fluid that fills the pericardial space, which may be due to infection or malignancy. A rapid accumulating effusion may lead to cardiac compromise.

CARDIAC TAMPONADE: Increased pressure within the pericardial space caused by an accumulating effusion, which compresses the heart and impedes diastolic filling.