Dad is an informal word for father. It’s a word like "pop" or "daddy." Some families have two dads, others have none. Sometimes a dad gets a “World’s Greatest Dad” mug on Father’s Day. Well done, sir.
If you’re talking about any ol’ dad, don’t capitalize it. There are dads all over the place. If you’re addressing your own father, basically using Dad as his name, do capitalize the “d.” The word dates from at least 1500, and it's believed to come from childish speech, or baby talk, since a very common first sound made by babies is "da."