It was a large war-horn and tipped with silver written in ancient characters. Boromir blew it before leaving Rivendell with the Fellowship, as he always did when setting out on a journey. He sounded it again in Moria, causing the Orcs and even the Balrog to pause in their advance. The third time he blew it was on the banks of Parth Galen, calling for help in defending Meriadoc Brandybuck and Peregrin Took. This last call, in addition to being heard by Aragorn, Legolas, and Gimli, was faintly heard by his brother Faramir and his father Denethor II. The horn was broken into two pieces during Boromir's last fight with the Orcs and Uruk-hai of Saruman at Amon Hen.
An heirloom of the house of the Stewards of Gondor, the Horn of Gondor had been carried by the eldest son for generations. The horn was crafted before the line of the Kings of Gondor was broken, when "Vorondil the Hunter father of Mardil Voronwë hunted the wild Kine of Araw in the far fields of Rhûn. “Made from the horn of "the wild ox of the East," the story surrounding the horn said, "if it be blown at need anywhere within the bounds of Gondor, as the realm was of old, its voice will not pass unheeded."
The Horn of Gondor was not present in Faramir's vision of his dead brother in the boat from Lórien. However, when Gandalf and Pippin arrive in Minas Tirith, the broken horn was being held by Denethor II, Boromir's father.
In Peter Jackson's Adaption of The Lord of the Rings The Fellowship of the Ring movie in the Extended version after Elrond bids the Fellowship farewell, the blowing of the Horn of Gondor by Boromir is cut from that scene and is not heard or seen in that extended scene of the film when the then newly formed Fellowship of the Ring take their leave of Rivendell. When the Fellowship are escaping into Eastern Moria running down the crumbling stairs and are pursed by the Balrog Boromir blowing of the Horn of Gondor is cut as well.
The only time it is featured and heard in the film is at the Battle of Amon Hen before Boromir dies.