In Generation I (when the item was known as the Exp.All in English), if it is in the Bag, Exp. All halves the experience and stat experience earned by the player's Pokémon that participated in the battle in order to distribute experience and stat experience to all party members.
Half of the earned experience and stat experience is distributed evenly among party members who participated in the battle. Then, due to a bug, experience and stat experience equal to the amount received by a single participating party member is distributed evenly among all party members. If only one of the player's Pokémon participated in the battle this amount is equal to half of the total, functioning as intended; however, if multiple Pokémon participated, some experience and stat experience that would have otherwise been earned is lost.
If any Pokémon in the party holds an Exp. Share, the total experience and stat experience earned when an opposing Pokémon faints is split in half, with 50% evenly distributed between all Pokémon who participated in the battle and 50% evenly distributed between all Pokémon holding an Exp. Share (rounded down to the nearest whole number).
When the Pokémon holding this item receives experience, the amount of Exp. Points gained is shown twice. Furthermore, a Pokémon whose Original Trainer is the player that both participates in battle and holds an Exp. Share will gain approximately 50% (due to rounding) of the experience twice, meaning that it does not receive 100% of the experience.
Generation III to V
If any Pokémon in the party holds an Exp. Share, the total experience earned when an opposing Pokémon faints is split in half, with 50% evenly distributed between all Pokémon who participated in the battle and 50% evenly distributed between all Pokémon holding an Exp. Share. A Pokémon holding an Exp. Share also gains all EVs (stat experience in Generation II) as if it had participated in the battle.
In Generation V, the Exp. Share accommodates the changes in the experience formula that incorporate the difference in levels between the defeated Pokémon and Pokémon gaining experience, which is factored in after splitting it.
Generation VI onward
Starting in Generation VI, it is now a Key Item. It can be turned on or off from the Bag menu in the overworld.
If it is enabled, the experience and EVs earned when an opposing Pokémon faints or is caught is awarded to all of the Pokémon in the party individually. Pokémon that do not participate in battle only gain 50% of the possible experience (but all of the EVs).
As the EXP.ALL, it allows the player to move an Energy card from a Knocked Out Active Pokémon to the Pokémon the EXP.ALL is attached to, then discard the EXP.ALL. As the Exp. Share, it has the same effect, except that it is not discarded after applying its effect.