How does weapon math work?

My ele picked up a weapon that had two -10 damage mods, also a +25 damage. What does that mean? Damage you don't take, along with damage you wield? Also, when you have the +20 damage added by "customization," how does that affect the -10 damage? If at all?

I think you need to either provide a screen shot or list out the weapon properties as they appear so we can figure out what you are refering to. Are these percents? Customization add 20% to damage of the weapon, not +20, HUGE difference.

None of this affects spells at all.