Eönwë was the banner-bearer and the herald of Manwë, and Chief of the Maiar along with Ilmarë. Eönwë was referred to as the 'greatest of arms in Arda', meaning that he was the best with weapons, though not necessarily the most powerful.
When the appeal of Eärendil reached the shores of Aman, it was Eönwë who first greeted him. When Manwë decided to heed the appeal, Eönwë was sent to Middle-Earth to fight the War of Wrath, leading the Vanyar.
When Morgoth the Black Foe was defeated, Eönwë took the two remaining Silmarils and held them for safekeeping. But at last the two remaining Sons of Fëanor took them and fled, yet Eönwë did not let them be slain.
In earlier conceptions of the legendarium (see: The History of Middle-earth), Eönwë, then called Fiönwë, was envisioned as the son of Manwë, but as the concept of the Children of the Valar disappeared from the Silmarillion tradition he was turned into Manwë's herald instead.