Explain the different types of damage to me!

I know that holy damage does 2x damage to undead, but does anyone have a chart or general rule of thumb about the differnt types of "elemental" damage. What do earth, air, fire and water exploit? Also, necros have shadow damage. What's the deal with this?

One other thing, how do the elemental damages (and holy/shadow) work regarding armor penetration. Does it ignore it?

For example, rangers have a spell that converts all damage to cold damage. When would this be ideal to use? (I haven't run into any fire elementals yet...)

Holy damage doesn't ignore armor per say, but will always do the damage it says (X 2 versus undead). So in a way, it sort of does, but not quite.

As far as elemental damage... I know fire owns ice guys...