(covers information from several alternate timelines)
"Then what is the purpose of your journey?"
A being who described himself as a Traveler, but whose real name he claimed was unpronounceable by Humans, was a humanoid from a mysterious species, encountered by Starfleet during the 2360s. Reported to be from Tau Alpha C, he himself claimed to be a traveler on a journey to experience "[our] reality". He also claimed that while by his understanding he was not from another time, he might be according to Starfleet's understanding of time. (TNG: "Where No One Has Gone Before", "Journey's End")
The Traveler had certain abilities, like the ability to act like "a lens which focuses thought", which allowed him to alter space, time, and warp fields with the power of his mind. He could phase out of time and dimension and move between planets and starships. These abilities were based on his ability to focus the energy of thoughts and in his advanced understanding of the nature of reality. According to The Traveler, thought was the basis of all reality. In addition, Deanna Troi noted that his presence could not be detected with the empathic abilities of Betazoids. He could also shapeshift to appear as humanoids from other species with different clothing. (TNG: "Where No One Has Gone Before", "Journey's End")
Around 2364, The Traveler had been trading his understanding of propulsion for passage on Starfleet vessels. This was the first time his species visited this culture. He claimed to be motivated by curiosity, his stated goal being experiencing "your reality." During these travels, he became an assistant to Starfleet propulsion expert Kosinski, who claimed to have discovered a revolutionary new method of warp drive. Although some of his theories did have merit, Kosinski's formulas turned out to be bogus, as Wesley Crusher discovered that it was actually Kosinski's assistant, The Traveler, who was the driving force behind the propulsion method. Things went awry, however, and the USS Enterprise-D was catapulted millions of light years to the galaxy known as M-33. Later, while attempting to return back to the Milky Way Galaxy, the Enterprise instead traveled to the end of the universe, a billion light years away. The Traveler collapsed in exhaustion and was taken to sickbay, where he revealed the secret of his abilities. He eventually returned to engineering, where he helped return the ship and crew home to the Milky Way Galaxy.
The Traveler noticed something special about the young Wesley Crusher, comparing him to a young Mozart. Inspired in part by this, Captain Picard promoted Wesley to an acting ensign in the traditions of Starfleet. The Traveler played a significant role in the destiny of Wesley Crusher. (TNG: "Where No One Has Gone Before")
In 2365, when the Enterprise encountered a duplicate version of Captain Picard, apparently transported six hours back in time, the contemporary Captain Picard briefly suspected The Traveler as a cause of this anomaly - noting that The Traveler was capable of movement through time using the power of his mind. Commander Riker, perhaps recalling The Traveler's largely benign nature, did not share this suspicion. (TNG: "Time Squared")
The Traveler was next encountered by the Enterprise crew in 2367, after a "warp bubble" experiment by Wesley Crusher went awry and trapped his mother Beverly Crusher in an alternate reality. Due to Wesley's increasing realization of his powers, the Traveler was drawn to him and sensed the predicament. He appeared and directed the crew on her rescue. He needed Wesley's help in opening a doorway to the other reality and had to convince him he had the ability. They succeeded in rescuing the elder Crusher, while the younger Crusher continued his maturation. (TNG: "Remember Me")
In 2370, The Traveler appeared incognito as Lakanta, a villager on Dorvan V, to Wesley Crusher. Lakanta, who seemed to have intimate knowledge of Wesley, directed him to seek the answers to his troubled destiny. Wesley was, at the time, quite uncertain about his future in Starfleet. The Traveler guided Wes to the "habak" where Wesley had a vision of his deceased father telling him that his destiny lay somewhere other than with Starfleet. The cadet heeded the advice and resigned from Starfleet, after which in the middle of a riot he managed to dissociate himself from time; it is then that The Traveler revealed his true identity and promised to mentor the young man on his new journey. (TNG: "Journey's End")
The Traveler was played by actor Eric Menyuk.
Menyuk was a finalist for the role of Data, and was given the recurring role of The Traveler as a consolation prize. (Starlog, December 1988, Issue 137, p. 54; Star Trek: The Next Generation Companion, 2nd ed., p. 36; Star Trek Encyclopedia (3rd ed., p. 520))
According to the Q Continuum series, The Traveler even has a significant reputation among the Q Continuum; when the female Q learned that Wesley had been chosen by The Traveler, she was almost impressed by this fact.