Power is a cumulative statistic that represents how well established you are, as well as how much of a threat you can be perceived to be. Essentially it is a rating system of how strong a user is, that allows you to size up your opponent quickly without having to do scans or track a user's movements.

Score is based on your power, crystals and relics. Below is how it is calculated:

1 crystal = 20 score
1 power = 10 score
1 relic = 500 score

Power is calculated based on your current workers, units and conquers:

25 workers = 1 power
10 infantry = 1 power
10 vehicles = 2 power
10 tanks = 3 power
1 conquer = 1 power

Hope that helps!
~Seth