In Generation I (called Exp.All), it shares experience gained from battle equally between all party members, but reduces the amount received by the battling Pokémon. They also gain stat experience shared equally between all party members.
From Generation II to Generation V, it is a held item that shares experience gained from battle between Pokémon that participate in battle and the holder. If the holder also takes part in the battle it will receive 75% of the experience, while the rest will receive only 25%. The holding Pokémon also gains all EVs (stat experience in Generation II) that would be gained by defeating the opponent.
In Generation II, when the Pokémon holding this item receives experience, the amount of Exp. Points gained is shown twice. If a Pokémon with the same original Trainer as the player is sent into battle with Exp. Share held, that Pokémon will receive approximately 50% of the experience (due to rounding) twice, which will not equal 100%.
In Generation V only, the experience formula was changed to incorporate the difference in levels between the defeated Pokémon and Pokémon gaining experience; this is factored in after splitting the experience.
From Generation VI onwards, it is a Key Item, and experience and effort values are given to all of the Pokémon in the party individually. Pokémon that do not participate in battle will only gain 50% of the possible experience.
Using a Lucky Egg doubles the experience points earned by the player for the next 30 minutes. They can be purchased from the in-game shop with PokéCoins from the start of the game. Some are also given for free when the player reaches certain level milestones.