Star

In the Angry Birds series, Stars are used as a format of a ranking system. When the player finishes a level, he/she is awarded one to three stars, with one being for simpliy finishing the stage, two for finishing the stage while also having a good score, and three for finishing the stage while also obtaining a hard-to-obtain score for that level, or utilitising a complex tatic to finish off the pigs using fewer birds, and with more damage to the structure itself. The difficulity for obtaining a three-star ranking varies with each level.

Format
When a stage is first unlocked, there is no writen score or stars collected. As stated above, to collect stars, the player must finish the level, and based on his/her performance, he/she will be given a mininum of 1 or maximum of 3 stars for the level. If the player is able to do this for all of the levels offered in an episode, a Golden Egg is given to the player. These variants of Golden Eggs are the most difficult to obtain.

In the original Angry Birds the Three-Star Golden Eggs unlock a minigame that if it's functions are done in a specific way, he/she will receive a Golden Egg Star. Angry Birds Seasons' Three-Star Golden Egg levels are actual levels that are played like any other level in the series. In version 2.0.0 of regular Angry Birds and version 2.5.0, the star format changes to that of Angry Birds Rio's.

Trivia

 * Many other physics-based iOS games, such as Cut the Rope and Save Toshi use the same, or simliar 3-star system. However, the former allows the player to end the level with a zero-star ranking, and the latter allows players to get a three-star-plus ranking.
 * Reseachers found that before the release of the original Angry Birds, only 20% of all games were rated with the 3-star score system. After the release of it, nearly 80% of all games were rated with the same or similar scoring system. (Similars are the likes of Asphalt 6, which placing was rated with 3, and achivements were 2, meaning that this is a 5-star score system.)
 * In the Angry Birds Board Games, there's a star piece.

Angry Birds Friends
You can get background your bird. Bronze need 100 stars, Silver 200 stars and Golden 400 stars.