Energy - Energy allow you to do missions and some events. Every time you do a quest step you spend Energy points. This is the only stat you'll need early in the game.
Attack - Attack allows you do fight other players and do some events. Every time you attack another player you spend Attack points. Increasing your Attack will not make each card's attacks stronger, it will just allow you to use more and higher power cards when you attack a player.
Defense - Defense allows you to defend yourself from other players. Every time you are attacked by another player, you spend Defense points.
Spending points in Energy, Attack, or Defense increases the maximum amount of points you can have in that category. All points recover over time up to your maximum level. You get stat points to spend from leveling up, finishing quests, or adding Team members.
Attack - A card's attack score determines how much damage it will do when you attack using that card.
Defense - A card's defense score determines how much damage it will do when you defend using that card.
Power Required - The number of player Attack or Defense points it will cost to use this card in a battle.