An Exp. Share is a held item that grants experience to the Pokémon holding it, even if they don't participate in battle, as long as they are not fainted.
Pokémon without participation receive 50% experience, and participating Pokémon gain full experience and EVs. Multiple Exp. Shares can be used simultaneously without interference. When an opposing Pokémon faints, the experience is evenly split between participating Pokémon and those holding an Exp. Share.