Each card belongs to certain element:
Water,
Fire,
Air,
Earth.

There are three degrees of damage that cards can deal to each other.
x 1 — regular damage (equals to card’s power)
x 1.5 — critical damage (power times 1.5)
x 0.5 — slight damage (half of power)

Elements inflict critical damage to each other in accordance with ancient canon: water puts out fire, fire burns air, air erodes earth and finally earth displaces water. In the same time reverse damage is halved. E.g. fire cards deal only half of regular damage to water cards.