Difference between Wenn and Ob

“ob”    “whether” or “if” . Used when talking about uncertainty or a decision between alternatives.

• Ex_: “Ich weiß nicht, ob ich kommen kann”_ means “I don’t know whether I can come.”

“wenn”    “when” or “if”. In the sense of conditions or specific situations. 

• Ex: “Wenn es regnet, bleiben wir zu Hause” means “If it rains, we stay at home.”