There are tons of potions to resist the different elements but what does Resist Magika do?? Is it a catchall.. will it also help to resist elements or is it against something else entirely.. like illusion or something?

Im pretty sure it resists all schools.

I could be wrong though, need more opinions to back my claim up.

It helps your character resist offensive spells. Max up to 85%, never immune.
Here you go: http://www.uesp.net/wiki/Skyrim:Resist_Magic
Hope that helps

It says it resist Magik damage I'm wondering if thaty just means spells that drain your magika. It doesn't say anything about the elements.

It's a catch all resist. It's usually lower than individual element types as it works for all (ie. a potion of fire resist might give 50%, whereas a similar level /skill magic resist would give 35%).