The Great Barrier was an immense energy field of unknown properties surrounding the galactic core of the Milky Way Galaxy. As of 2287, the Barrier was thought to be impassable, as no probe had ever returned.
In that year, the USS Enterprise became the first known vessel to penetrate the barrier, having been commandeered by the renegade Vulcan Sybok, who was on a quest to discover the mythical planet of Sha Ka Ree. Claiming to have been given a vision by God himself, Sybok mentally manipulated the Enterprise crew into approaching the center of the galaxy, believing that God resided on Sha Ka Ree on the other side. Although Captain Kirk believed that the Enterprise would be destroyed in the attempt, Sybok maintained that the danger posed by the Barrier was simply an illusion, an expression of the universal fear of the unknown. Sybok was ultimately proven correct when the ship passed through with no ill effects. In addition, the Enterprise encountered a planet on the other side of the Barrier, seemingly confirming Sybok's vision.
A malevolent alien entity was found on the planet, imprisoned by the Barrier. It impersonated the God Sybok was expecting to find in an attempt to manipulate Sybok into providing it with a starship with which it could escape. Upon discovery of this fact, Sybok attempted to oppose the entity, as the Enterprise crew fired on its location with photon torpedoes. Although this resulted in the death of Sybok, the alien entity survived. The entity was also assaulted with disruptor cannon fire by a Klingon Bird-of-Prey, which had pursued the Enterprise through the Barrier. (Star Trek V: The Final Frontier)
The Great Barrier, and the planet within, would seem to have appeared only at most eighteen years prior to 2287. In the episode TAS: "The Magicks of Megas-Tu", the USS Enterprise explored the center of the galaxy. At that time the center was discovered to be a point between dimensions surrounded by a matter-energy whirlwind. Presumably the point between dimension had disappeared since no one expected to find it eighteen years later. There also presumably was no Great Barrier back in 2269, since the Enterprise would have to have crossed it.
The fact that the Barrier and center of the galaxy were only some 6.7 hours away from Nimbus III, in the Neutral Zone, at warp 7 is inconsistent with how fast the warp speeds have been established to be later on in Star Trek after Star Trek V: The Final Frontier.
In Star Trek: Star Charts (p. 11), the Great Barrier is depicted as a spherical energy field engulfing the whole galactic core and all the stars within. The diameter of the Great Barrier is approximately 15,000 light years. The planet encountered in Star Trek V: The Final Frontier is not depicted to be at the center of the galaxy. Instead, it is at the outer edges of the energy field, some 7,000 light years away from the center. The point between dimensions, where the crew traveled to in "The Magicks of Megas-Tu", is labeled as a massive black hole called Sagittarius A at the center of the galaxy. This location for the planet would be inconsistent with the movie, but makes it possible for both accounts of the center of the galaxy to coexist, though Sagittarius A* (pronounced "A-star", as it was called in the real world), a supermassive black hole, was not compatible with the creation point phenomenon that was seen in "The Magicks of Megas-Tu". The galactic barrier is connected to the Great Barrier. The Great Barrier is stated to be a related, but distinct energy field.
According to the novel Q-Strike, The Great Barrier was created 500,000 years ago by the Q Continuum to imprison the malevolent entity known as The One to the planet for its crimes. According to the same novel, the galactic barrier was also created to protect the Milky Way from 0 (Zero), one of the Q Continuum characters.