On the same unit screen that shows the resistance, it shows the percentage a resistance will reduce the damage by if you hover over the number. I believe it is a logarithmic scale(?), with each additional point reducing the damage less than the previous one. Someone made a chart based on people reporting the resistances at various numbers:
https://docs.google.com/spreadsheet/ccc?key=0AvzPRcaoz7qIdDF4Wm9Idng3U1ZCcjhRMkNtZXVtdFE#gid=0
You CAN hit -100%, it requires around 300 resistance. 90% resistance arrives around 160ish.
As for unit power, that is the base damage a unit can inflict. All multipliers are based off of the unit's power. So, when you purchase upgrades like Enchanted Weapons (+20% Elemental Damage), the 20% is based off of the units power, not its overall damage shown next to to the units portrait.
This makes Level ups like Sniper (+20% Unit Power) more valuable than Marksman (+20% Missile Attack). It's still good to eventually get both, however.
