These two words are of medical jargon. When we consult a doctor, The doctor may be able to forecast what the problem is, just with the symptoms we told. So he is predicting a disease well before the actual examination.This is prognosis ( pro=before, gnosis=knowledge)

If he does all the tests, and decides what exactly the disease is? Then he is examining between two or more diseases to decide the exact one. This is diagnosis (dia=between,gnosis=knowledge)