(covers information from several alternate timelines)
Sybok was the firstborn son of the Vulcan ambassador Sarek and a Vulcan princess, and was the elder half-brother of Starfleet officer Spock. A revolutionary, Sybok sought experience and knowledge which were forbidden by Vulcan belief.
After the death of his mother, Sybok was raised by Sarek as a half-brother to Spock. As a young student, Sybok was considered to be exceptionally gifted, possessing great intelligence, and it was assumed that he would one day take his place among the great scholars of Vulcan. However, Sybok rejected his logical upbringing and chose to embrace emotion like his ancestors, believing that the key to self-knowledge was emotion, not logic. When Sybok encouraged others to follow him, he was banished, never to return.
During his studies, Sybok received visions given to him by an entity whom he believed to be God himself, and began a journey for a place in Vulcan mythology known as "Sha Ka Ree," from where all of existence was said to have originated. In 2287, Sybok's visions led him to Nimbus III.
During his travels, Sybok gained allies by asking the people he encountered to "share their pain" with him; he used the telepathic abilities inherent to his Vulcan heritage to help them relive traumatic experiences in their lives, allowing them to face and subsequently overcome their most deep-seated regrets.
These were very intense, powerful experiences that could sometimes be witnessed and shared by other people (as was the case with Kirk, Spock, and McCoy). The recipients of Sybok's "therapy" were always thankful to be healed of their psychological trauma, and would readily join Sybok's quest out of gratitude toward him. This allowed Sybok to amass a large group of followers who dubbed themselves the Galactic Army of Light, and to easily capture Paradise City.
In an effort to gain access to a starship, Sybok subsequently kidnapped three ambassadors to Nimbus III – St. John Talbot of the Federation, General Korrd of the Klingon Empire, and Caithlin Dar of the Romulan Star Empire – and successfully manipulated them into joining him. This eventually led to a failed rescue attempt by the crew of the USS Enterprise-A, wherein Sybok was reunited with his brother, Spock. Commandeering the Enterprise, Sybok had the crew set a course for the center of the galaxy, where he believed Sha Ka Ree existed, beyond the Great Barrier.
Although Captain Kirk insisted that the Enterprise would be destroyed attempting to penetrate the Barrier, Sybok was eventually vindicated when the ship was able to do so without incident, and discovered a planet on the other side. Traveling to the surface with Kirk, Spock, and Doctor Leonard McCoy, Sybok was eventually confronted with the fact that the being which had given him his visions was not, in fact, God, but rather a malevolent alien entity who had been imprisoned within the Great Barrier, and who had manipulated Sybok into providing him with a starship with which he might escape. Realizing that he had made a terrible mistake, Sybok confronted this being, sacrificing his own life to save the crew of the Enterprise. (Star Trek V: The Final Frontier)
Background information Edit
In the original story outline, Sybok was named "Zar".  The role of Sybok was written for popular actor Sean Connery; however, this did not come to be and the role was, ultimately, played by Laurence Luckinbill.
According to the Star Trek Chronology (2nd ed., p. 38) and the Star Trek Encyclopedia (4th ed., vol. 2, p. 349), Sybok was born in 2224, based on conjecture that Spock was born in 2230. Sybok is Spock's older half-brother despite the fact that Laurence Luckinbill was more than three years younger than Leonard Nimoy.
It was stated in TNG: "Sarek" that Sarek's first wife was from Earth. Clearly, this was intended to be a reference to Amanda Grayson and that Sarek was not married to a Vulcan princess. Admittedly, Star Trek V never explicitly says that Sarek ever married the princess, nor was it stated in "Sarek" that Amanda was Sarek's first Human wife, or how many previous wives he had, only that the Vulcan princess was Sybok's mother.
The novelization for Star Trek V and the novel Sarek both name Sybok's mother as T'Rea (β), a Kolinahr adept who had been bonded to Sarek in childhood. She rose to become High Master, whereupon – per Kolinahr tradition, requiring the severing of emotional ties – her marriage to Sarek was annulled, explaining why Amanda is considered his "first" wife. The novelization goes on to explain that Sybok was banished from Vulcan for violating the mind of a Watcher in the Hall of Ancient Thought in order to locate the receptacle of his mother's katra, and discover the location of Sha Ka Ree.