The Skirmish at Amon Hen was a battle between the Fellowship of the Ring and a small force of Uruk-hai, and was fought during the Third Age at Amon Hen. It resulted in the death of Boromir and the breaking of the Fellowship.
Frodo was given an hour alone to decide how the Fellowship would proceed in their quest. However, after an hour had long past, Frodo had still not returned. Boromir, who had earlier abandoned the remaining members of the Fellowship without their knowing, then returned. He recounted that he had seen Frodo an hour before, that he had tried to convince Frodo to bring the Ring to Minas Tirith, and that they then had an argument, resulting in Frodo putting on the Ring and vanishing. This story roused the suspicions of Aragorn, who guessed correctly that Boromir had not told the entire truth. At the conclusion of Boromir's story, Merry and Pippin immediately ran off to locate Frodo while Legolas and Gimli searched for the Ringbearer in a different direction.
Fearing that the Fellowship would be scattered and lost, Aragorn commanded Boromir to follow Merry and Pippin. Aragorn then instructed Sam to follow him to the Seat of Amon Hen, but Sam was unable to keep up with him. Thinking on his feet, Sam returned to camp seeing a boat seemingly push itself into the river. Realizing that an invisible Frodo was pushing the boat, Sam catches up to Frodo, convincing him to let him accompany him to Mordor.
Meanwhile, Aragorn, who had reached the Seat of Amon Hen, hears the sounds of Orcs and Boromir blowing his horn. He runs to Boromir's aid but arrives too late along with the battle-worn Legolas and Gimli shortly thereafter. Boromir fought valiantly, redeeming himself for trying to steal the Ring from Frodo, but fell to many arrows allowing Merry and Pippin to be captured. With Frodo and Sam heading off to Mordor on their own, Aragorn decides that the fate of the Ring is no longer in their hands, so he, Legolas, and Gimli set out to rescue Merry and Pippin.
Portrayal in Adaptations
In Peter Jackson's film the Uruk-hai are led by Lurtz, who mortally wounds Boromir by shooting him with three arrows, before engaging in a duel with Aragorn. Aragorn does not participate in any combat in the books. Merry and Pippin, aware of Frodo's intention to leave the Fellowship, willingly create a diversion to allow Frodo to escape -- they are captured moments after Boromir is incapacitated by Lurtz's arrows. Though Lurtz is decapitated by Aragorn, Boromir is beyond saving. As in the book, he admits to attempting to take the One Ring by force and begs forgiveness, lamenting that the strength of Men will fail and that Sauron may succeed. Aragorn swears that Minas Tirith will not fall, nor will the strength of Men fail. Boromir accepts Aragorn as his king, before passing away peacefully.