Chickens are mobs that first appeared in the Alpha version of Minecraft. They randomly lay Eggs (delay is usually 5–10 minutes) and when killed they drop 0-2 feathers. They are passive mobs and will flee from the player when they are attacked.
After 1.8 Beta was released, chickens can now drop Raw chicken to fill up the player's food bar. However, if you eat it raw, you have a chance to be poisoned much like Rotten Flesh. They also drop feathers which are used to craft arrows.
This cycle will go on for a very long time, since this trick is effective. Just grab a handful of eggs, put them in a hole, and let them grow. Then let the chickens lay more eggs, upon which you kill them and repeat the process. Using this strategy, you won't have to go around very far to hunt them. This also allows you to gather large amounts of feathers and raw chicken at a time.