Biography: From Wikipedia
Aberforth Dumbledore is Albus Dumbledore's brother. He was the younger by some three years, and the lesser skilled of the two; Albus even notes that he is uncertain if Aberforth can read. As such, he was usually left in the background while his brother basked in glory. He became the caretaker of his younger sister Ariana, but got into an argument with his brother and his brother's friend, Gellert Grindelwald, which resulted in Ariana's accidental death. At Ariana's funeral, he publicly confronted Albus and broke his nose. He has a strong affinity for goats probably because of the goat farm that the Dumbledores owned during Aberforth's youth.
Rowling admitted in an interview, in response to a question by a reader, that Aberforth is the bartender Harry saw at the Hog's Head pub in Hogsmeade. There are hints that Aberforth is the bartender appearing in Harry Potter and the Order of the Phoenix when Harry noted a "strong odour of goats" in the Hog's Head Inn (in the film version the bartender was seen with a goat), and that the barman looked familiar. Harry had earlier seen a photo of the Order of the Phoenix, which included Albus and Aberforth, and they look similar. Albus mentions in Harry Potter and the Goblet of Fire that Aberforth was arrested for "performing inappropriate charms" on a goat. Another link to his goat history is in Harry Potter and the Deathly Hallows when he tries to convince the Death Eaters that Harry's patronus, a stag, was his own, which takes the form of a goat. In a Pensieve memory in Harry Potter and the Half-Blood Prince, Albus informs Voldemort that he is "friendly with the local barmen" when able to inform Riddle that he knows his followers are near by waiting for him. Aberforth is also seen at Albus' funeral at the end of that book.
However, it was not until Deathly Hallows that Aberforth was named directly. He plays an important role by taking Harry, Ron, Hermione into his bar before the Death Eaters can arrest them. Aberforth later reveals to the trio some facts they did not know about the history of the Dumbledore family. In addition, it is also revealed that he purchased Sirius Black's two-way mirror from Mundungus Fletcher in the previous book. Harry briefly glimpses Aberforth's eye in its counterpart's remaining shard while being prisoner in Malfoy Manor. Using the mirror to watch over the trio, Aberforth sends Dobby to rescue them and some other prisoners from the Manor.
Aberforth allows the resistance fighters to use a secure passageway from the Hog's Head to the Room of Requirement via Ariana's portrait. The same passage was used to evacuate underage students from Hogwarts, and, according to Neville, it was also used by members of Dumbledore's Army to get food when they were living in the Room of Requirement. Aberforth left the Order, believing the war against Voldemort was lost. However, he quickly joins the Battle of Hogwarts, and is last seen stunning Augustus Rookwood. According to Rowling, Aberforth survived the battle, and is still "at the Hog's Head, playing with his goats".