(covers information from several alternate timelines)
"Vulcan mind melds: utter foolishness. Anybody with an ounce of sense wouldn't share his brain with someone else; would you? I certainly wouldn't."
The Vulcan mind meld was a telepathic link between two individuals, allowing for the exchange of thoughts, thus in essence allowing the participants to become one mind. (TNG: "Sarek") It was a psionic technique for "synaptic pattern displacement". Normally it was employed only by Vulcans. It was a deeply personal thing, part of the private life, and generally not used on aliens, although cases were known where the mind meld was initiated between a Vulcan and a non-Vulcan. (VOY: "Meld", TNG: "Sarek") Those in a meld shared consciousness in a kind of gestalt. (DS9: "The Passenger")
Physical contact was required. The initiator placed the tips of his fingers at key locations on the head of the other participant. This allowed him to place pressure on key nerves and blood vessels to facilitate the link.
The fact that most Vulcans who performed the meld touch the other participant in different manners, reciting varying formulas, implies that it was only ritual behavior, helping to concentrate on the task, leaving mere physical contact being the ultimate prerequisite to initiate the meld. If successful, the link resulted in a merging of both minds, essentially creating a single consciousness in two bodies.
It is noteworthy that the meld could also be used by its initiator to probe another person's mind, while the melder him/herself shielded his/her mind from being read by the other participant, resulting in a rather one-sided psionic contact. (Star Trek VI: The Undiscovered Country) Only the subject in physical contact with the initiator participated in the meld; others in the vicinity are not affected.
Some species are immune to Vulcan mind meld and can resist it, shielding their mind from the attempted probing, including Cardassians, who can achieve this thanks to their intense mental training since childhood. (DS9: "The Maquis, Part II")
A mind meld could even be used to transfer the entire personality or "soul" (known to Vulcans as the katra) of an individual into another body. A special, appropriately prepared receptacle, such as a katric ark could also be used. Though Vulcans (and Trill, through the zhian'tara ritual) did this psionically, a number of people and races were able to duplicate this feat with technology, such as the people of Sargon's planet, Janice Lester, Dr. Ira Graves, Rao Vantika, and Tieran. (TOS: "Return to Tomorrow", "Turnabout Intruder"; Star Trek III: The Search for Spock; TNG: "The Schizoid Man"; VOY: "Warlord"; DS9: "The Passenger", "Facets")
Melding carried some risk. It could be physically debilitating for both parties. The pressure changes could potentially aggravate existing conditions. The melding resulted in some loss of identity, and could be difficult to break, especially when the subject's mind was powerful or dynamic. Aftereffects could be treated with the drug lexorin.
At the conclusion of the meld, each mind retained some knowledge of the other.
It was possible to force a mind meld on an unwilling subject, but most Vulcans would be loath to do so, unless under the most dire circumstances. An exception may be the Vulcans of the mirror universe, whose ethical constraints, shaped by the savagery of their environment, were far different.
During the 22nd century, mind melding was believed to be an ability only a minority of Vulcans were born to. Because of the apparent intimacy of melding, Vulcans during this era considered it a deviant practice defiant to the ancestral teachings of their society. As a result, other Vulcans considered those who were natural "melders" to be outcasts. (ENT: "Stigma")
An improperly trained melder who initiated a mind meld can cause a degenerative neurological disorder known as Pa'nar Syndrome to develop in the meld recipient. If left untreated, the disorder is fatal. Vulcan medicine of the mid-22nd century believed that there was no cure; however, the disorder was known in Surak's time, as was its cure: a corrective meld performed by an experienced melder. It is likely that the ignorance of this cure was due to the widespread prejudice against melders, a prejudice that was not shared by the Syrrannite group. (ENT: "Fusion", "Stigma", "Kir'Shara")
Another side-effect is the transfer of emotion. When Spock Prime melded with the James Kirk of the alternate reality, both seemed emotionaly effected by the experience and Spock Prime apologized, explaining that emotional transference is a side-effect of the melding process.
In 2151, the v'tosh ka'tur Tolaris initiated a mind meld with Subcommander T'Pol against her will. Since Tolaris had not been properly trained in the use of his mental abilities, this meld caused T'Pol to develop Pa'nar Syndrome. (ENT: "Fusion", "Stigma")
In 2154, Vulcan Ambassador Soval, defying the taboo of his day, melded with a comatose Corporal Askwith to learn who bombed the United Earth Embassy on Vulcan, deciding it was worth the risk upon discovering that the evidence implicating the Syrranites was fabricated. For this offense, Administrator V'Las (who was actually behind the bombing) relieved Soval of his post. (ENT: "The Forge")
In November of 2154, T'Pol initiated her first mind meld on the starship Enterprise NX-01. In an attempt to ultimately discover the location of Doctor Phlox, T'Pol melded with Ensign Hoshi Sato. (ENT: "Affliction")
In January of 2155 of the mirror universe, T'Pol, via a mind meld, implanted a telepathic "suggestion" into the mind of Commander Charles Tucker. Under this influence, Tucker sabotaged the power grid of the ISS Enterprise in order to disable a Suliban cloaking device. After doing this, T'Pol melded with Tucker again, erasing his memories of his actions. (ENT: "In a Mirror, Darkly")
In an alternate reality, future Spock melded with young Kirk in 2258 to explain how he and Nero came from the future. Furthermore, Spock mind-melded with a Romulan to find out where Captain Pike was being held. (Star Trek)
In 2266, on stardate 2715, Spock melded with Simon Van Gelder, then suffering from generalized synaptic damage, to learn whether Van Gelder's allegations about Tristan Adams were true, or delusions. Spock warned Van Gelder that the pressure changes could be dangerous. (TOS: "Dagger of the Mind")
The same year, on stardate 3192, Spock used what might have been a variation on a mind meld to convince a guard on Eminiar VII to open the door to the room where the landing party was imprisoned, facilitating their escape. (TOS: "A Taste of Armageddon")
Also in 2267, McCoy became the victim of an unwilling mind meld, when the mirror universe Spock forced information from him about the nature of the transporter accident that brought four USS Enterprise officers to the mirror universe. (TOS: "Mirror, Mirror")
On stardate 4657.5, Spock attempted a meld with Kelinda, a Kelvan, through a few feet of solid rock. Although the attempt was thwarted by Kelinda with prejudice, Spock was able to get an impression of the Kelvan's true non-humanoid form.
In 2268, on stardate 4385.3, Spock melded with Kirk, McCoy and Scott to ensure they believed the bullets fired at them from Virgil Earp, Morgan Earp, Wyatt Earp, and Doc Holliday (at the OK Corral) were unreal and, therefore, unable to harm them. (TOS: "Spectre of the Gun")
The same year, Spock melded with Kirk, who was suffering from retrograde amnesia, consequential to the misuse of a Preserver artifact. Although Spock had some difficulty emerging from this meld, he was able to successfully restore Kirk's memory. (TOS: "The Paradise Syndrome")
Later in 2268, Spock was driven mad by the sight of Ambassador Kollos. Miranda Jones, a Human telepath trained on Vulcan, used a meld to restore his sanity by making him forget what he'd seen. (TOS: "Is There in Truth No Beauty?")
In 2273, when the Enterprise was inside V'Ger, Commander Spock entered a section of V'Ger where it stored 3D images of objects that it encountered through its travels. He entered this area via thruster suit and subsequently encountered a large representation of Lieutenant Ilia with a pulsing sensor on her neck. Believing it to have some special meaning, he used a mind meld on it. Spock was overwhelmed by the information from the mind meld, and was flung back unconscious towards the Enterprise. (Star Trek: The Motion Picture)
In 2285, Spock used a mind meld to transfer his katra to McCoy before sacrificing himself to restore warp power during the Battle of the Mutara Nebula. (Star Trek II: The Wrath of Khan) Upon the Enterprise's return to Earth, Sarek mind-melded with Kirk to discover the fate of Spock's katra. (Star Trek III: The Search for Spock)
After time-traveling to 1986, Spock mind melded with the humpback whales George and Gracie to inform them of the crew's plan to bring the whales back with them to the year 2286 to answer a mysterious probe that threatened Earth, reasoning that they had to confirm that the whales were willing to help them or they would be no better than those who made the whales extinct in the first place. (Star Trek IV: The Voyage Home)
In 2370, Sakonna, a Vulcan member of the Maquis, attempted a mind meld on Gul Dukat in an attempt to learn the location of illegal Cardassian weapons in the Demilitarized Zone. (DS9: "The Maquis, Part II")
In 2371, Tuvok melded with crewmate Lieutenant Tom Paris after Paris was accused of the murder of Banean scientist Doctor Tolen Ren. The meld allowed him to prove that Paris had been framed in an elaborate plot to provide the Banean's enemies, the Numiri, with critical information, when his analysis of the implanted memory revealed crucial anomalies that proved Paris couldn't have committed the murder. (VOY: "Ex Post Facto")
In 2372, Tuvok melded with the sociopath Betazoid Lon Suder in an attempt to gain a better understanding of Suder's violent impulses. While the meld provided Suder with a measure of emotional control, it also temporarily released Tuvok's more violent and primal urges. (VOY: "Meld")
Later that year, he melded with an alien named Guill in an investigation to prove B'Elanna Torres' innocence. Guill was from a society where violent thoughts were illegal. Guill was a collector and trader of such thoughts. Tuvok proceeded with the meld under the pretense that only images would be exchanged, but he also grabbed and choked Guill in the meld to get him to submit. (VOY: "Random Thoughts")
In 2376, Tuvok performed an extended technique of the mind meld, the "Bridging of Minds", on Captain Janeway and Seven of Nine, forming a bond between the two individuals, who were otherwise unable to telepathically link. (VOY: "Unimatrix Zero")
In the novelization of The Undiscovered Country featured a slightly different take on the forced mindmeld between Spock and Valeris. As Spock probed her mind, he stopped just short of breaking her will and gave her the choice to willingly help him. The choked sobs by Valeris was her gratitude and grief at being given the choice.
In the novel Memory Prime, the mind meld was used as a form of combat between Spock and a Romulan assassin.
In the William Shatner novel The Return, Spock performed a meld with Picard and Kirk in an attempt to free Kirk from the brainwashing program that had been implanted in him by the Borg/Romulan Alliance; at the time, Spock said that such a thing had never been done, but it is possible that the Bridging wasn't well-known among Vulcans.