Gimme some onion knowledge

I cook a lot, and just about everything I do cook has onions in it. They make everything better.

But the other day I was standing at the grocery store in the produce section looking at the array of varieties of onions they had when I realized I really don't know if there's reason to pick between yellow, white, sweet, and whatever else they have. I guess I just always usually pick yellow onions because my dad taught me how to cook and that's what he always used.

So, should I be using yellows for some dishes and white for others? Is one better for smothered venison and another for say, Mexican food?

Are sweet vidalia onions better than either?

Gimme the down low.