The Exp. Share is a held item introduced in Generation II (Exp. Share in Generation 1 existed but was not a held item and divided exp amongst your entire party). When the player gives an Exp. Share to a Pokémon in their party that isn't first in the lineup and battles, part of the experience points gained by the battling Pokémon is given to the one holding the Exp. Share. However, the Pokémon who is battling does not gain the regular amount of experience; it gains half of the normal amount. There is usually only one Exp. Share found in each game, you can however obtain more by cheating, winning one at the lottery, trading Pokémon holding another Exp. Share, or, in Pokémon Black and White, receive a second one from the Pokémon Fan Club Chairman.
Any Pokémon actively in the battle receive half of the total experience. The other half gets split up among any Pokémon holding Exp. Shares. Example:
# of Pokémon with Exp. Share
Battling Pokémon receives
The Pokémon with Exp. Shares receive
100 Exp. points
50 Exp. points
25 Exp. points
Note: If the Pokémon that is holding the Exp. Share is the one fighting, extra experience is not gained and experience is not given to any of the other Pokémon in the party. The only item that raises the amount of XP gained is the lucky egg which can be found rarely being held by a wild Chansey.
Note: In Generation VI Pokemon who win a battle will receive 100 percent on the Xp and the rest of the Pokemon in your party will only receive 50 percent, rather than splitting all of the XP to your party.
After Generation V, the Exp. Share will be no longer as experience holding item then used as Key Item.