A regular card has three numbers - ATTACK, DEFENSE, and VALUE.
One person plays a card using its ATTACK. Let's say the card has the numbers "2 5 3" on it. This means he is attacking with 2.
The other person has to put up a card with a higher DEFENSE then the other person's ATTACK. This means the opponent has to put up a card with a 3 or higher DEFENSE.
Once again, the defender wins if his DEFENSE number is larger than opponent's ATTACK. If the DEFENSE number is equal or lower, the ATTACKER wins.
Whoever wins gains the other card's value as points - if the ATTACKER wins, he gains the value of the DEFENDER'S card, if the DEFENDER wins, he gains the value of the ATTACKER'S card.

Let's say someone just started playing and attacked with a "2 5 3"
Someone then defends with a card with stats "0 3 4"
The defender's DEFEND number - 3 - is larger than the attacker's ATTACK number - 2.
The defender therefore gains the VALUE of the card "2 5 3" which is the last number, 3.
The score so far is 3:0 for whoever won this round.

Then it is onto the next round, with the other person attacking this time.
There are also a couple ACTION cards which do not work as other cards. They have no value but only provide attack, meaning they will either score you points, or do nothing.
One to particularly keep note of is the SOUP, which kills any opponent.

A couple cards have special attributes, like lowering defense, extra points, and more, but they are rare.