V'Ger was a extraordinary machine entity encountered by the Federation in 2273. The entity's surrounding energy cloud was over two AUs in diameter and generated amounts of radiation rivalling the heliosphere of Sol. The luminescent cloud interior was measured to be a level of 12th power energy.
The size and power of V'Ger worried all in the Federation, when it was detected on a direct heading for Earth, at a sustained velocity of Warp factor 7. The interior of the spacecraft seemed to be capable of holding a crew of tens of thousands. Or, a crew of a thousand-each ten miles tall.
With the entity being over 54 hours from Earth, Starfleet rushed the refit of the USS Enterprise, so that it could be launched in time to intercept V'Ger. After penetrating the huge entity and traveling to the center, Admiral James T. Kirk and his crew encountered V'Ger, who proceded to send a probe that "absorbed" Enterprise navigator Ilia; with all of her memories and personality. "She" returned in the form of the now-deceased Lieutenant: wearing her casual attire and equipped with a transmitter/receiver (or, tranceiver combination). Soon after, the Enterprise crew discovered that V'Ger was actually the NASA probe, Voyager 6, which had been launched from Earth in 1999, entered a black hole, and achieved sentience.
V'Ger took its mission to heart and learn everything it could to extremes, as it absorbed everything it was exposed to, and having learned everything, wanted to return to Earth to impart its knowledge (by old-style radio waves) and join with its creator. Realizing that the fate of Earth was in their hands, Commander Willard Decker joined with the entity and programmed the information it needed to transmit its data. Once the two were joined, the entity evolved onto another plane of existence and headed for the Andromeda Galaxy. (TOS movie: Star Trek: The Motion Picture; TOS - Star Trek II Short Stories short story: "To Wherever")
After encountering the Borg, it was believed that the planet V'Ger landed on was the Central Node and that V'Ger was simply another branch of the Borg Collective that assimilated through the use of energy. (TOS novel: The Return)
However, the Vulcan scientist T'Uerell, after deciphering the secrets of the Borg Collective, uncovered evidence that it was V'Ger who first created the Collective to serve as heralds in its search for its creator. However, the Borg strayed from their original purpose as the Borg Queen assumed more power. (ST video game: Legacy)
The evolution of V'Ger had some wide ranging affects on the rest of the Federation and beyond. On Daran IV, Dovraku and his "Faithful" viewed this as a Sign that the time had come to reinstate the Oracle of the People's control over the Fabrini. (TOS novel: Ex Machina)
On planet Mestiko, the defeat of this threat to Earth was used as proof that the Federation could have defended their world from the destructive effects of "the Pulse," and opted not to in order to subjugate the Payav race. (TOS - Mere Anarchy eBook: The Darkness Drops Again)
In the alternate timeline created by Nero, V'Ger was still in the Delta Quadrant as of 2258. It made telepathic contact with the Narada (seeing its sentient computer systems as a kindred spirit), summoning it from where it was moored in orbit of Rura Penthe. Nero was able to exploit this connection in order to use V'Ger's powerful computers to calculate where and when Spock would emerge from the black hole. Nero also learned somewhat more about the origin of the probe, namely that both it, and the Borg, are, "children of an ancient, unknown civilization." It is possible that this is a reference to the Caeliar. (TOS comic: "Nero, Number Three")