On the Island....

Boone became the protégé of John Locke, having initially followed him to hunt for Charlie and Claire after they had been kidnapped by Ethan Rom, then discovering and digging up the metal hatch to a mysterious pod-- the existence of which Locke swore him to secrecy, claiming that the others were not ready to know about it, and instead to tell them they were "hunting for boar" . Locke was also instrumental in changing Boone's perception of his relationship with his step-sister. In Deus Ex Machina, Locke has a vision-dream about Boone, in which he repeats the phrase, "Theresa falls up the stairs; Theresa falls down the stairs." Boone reveals that Theresa had been his nanny as a child, upon whom he took out his frustration at the absence of his mother. She fell to her death down the stairs in his house while responding to his summons. The same dream led to the "sign" from the island of an small airplane crashed in the forest, which the two followed to discover a heroin runner's aircraft stuck in the trees.

Boone died, in Do No Harm, of wounds sustained while falling out of a tree in a crashed airplane in the previous episode.

Boone frequently wore a t-shirt with the Chinese characters, meaning 84. The three characters individually mean 8-10-4, two of which are the unlucky Numbers that frequently appear throughout the show. This is also the shirt that Boone wore in Locke's vision of him, and the shirt he wore when he died.
http://www.answers.com/topic/characters-of-lost

Boone's death was foreshadowed by his Star Trek / red shirts comment to Locke in (COWBOY). (noted by templeton at the fuselage)