Eonwë is a Maia in service to the chief Vala, Manwë in J.R.R. Tolkien's mythology. His portfolio is the banner-bearer or Herald of the Valar, and in that role he played the trumpet.
In that role, he led the attack on Morgoth at the end of the First Age. The text says that Eonwë is unsurpassed in all of Arda in "might of arms." This does not contradict the text saying that Tulkas is greatest in strength, as Tulkas uses no weapons.
At the end of the First Age, Eonwë greeted Eärendil on the shores of Valinor, and escorted him before the Powers of Arda. When Morgoth was captured, Eonwë guarded the Silmarils, despite a challenge from the two surviving sons of Fëanor, Maedhros and Maglor. When the brothers invaded his camp, Eonwë refused to allow the guards to kill the thieves.
Eonwë also accepted the surrender of Sauron, but Sauron recanted and refused to return to Valinor for judgement. How different the lives of the Edain would have been, if he had.
Eonwë is also expected to play a key role at the end of the universe, when the Great Enemy returns from beyond the bounds of the universe, and the Halls of Mandos is emptied to fight the Final Battle.