Clash of Clans uses a trophy system to match you with comparable opponents, and places you in a Trophy League when you attack.

You gain trophies by successfully attacking or defending against other players. The amount of trophies you win or lose is based on the difference between you and your enemy. Remember:

• If you win an attack or defend against a player with a higher trophy count than you, you will be rewarded with a larger number of trophies
  
• If you win an attack or defense against an enemy with less trophies than you, you’ll be rewarded with a smaller amount of trophies
  
• If you lose an attack or defense against an enemy with less trophies than you, you will lose a large amount of trophies
  
• If you lose an attack or defense against an enemy with more trophies than, you will lose less trophies