Telepathy is the communication of messages or thoughts directly from one mind to another without use of ordinary vocal and auditory mechanisms. Telepathy exists across space; it has appeared in various forms among various races.
A touch telepath can communicate directly with the mind of another being, but only if in physical contact. The Vulcan mind meld is a form of touch telepathy (although there is more to it than that).
Some telepaths can exchange thoughts only with others of their own species, some can send but not receive, and some can receive but not send. Betazoids, for one example, can send and receive the thoughts of other Betazoids, but can only receive thoughts and feelings from alien minds.
In some races, telepathy is present in all healthy individuals (those not born with congenital problems and not the victims of accidents or disease). In other species, such as Humans, telepathy appears only infrequently. And certain species, notably the Ferengi, are never telepathic. Often such species cannot be read even by telepaths normally capable of reading alien minds.
Telepathy makes possible some forms of interaction that would otherwise be impossible. The Medusans are an incorporeal race so ugly (or perhaps merely so alien) that the sight of them drives humans insane. Interaction with them was very limited, until 2268, when Miranda Jones established a telepathic link with Ambassador Kollos, proving that it could be done.
Extraordinarily powerful telepathic species exist. They are rare, but are extremely formidable, as they can immerse other minds in a realty of their own fabrication, much like a holodeck, but where the telepath makes all the rules. The xenophobic Melkot used their telepathic powers to punish aliens who encroached upon their space without permission (which, prior to 2268, they never extended).
Under rare circumstances, individuals can gain telepathic powers. In 2264, exposure to the galactic barrier altered Gary Mitchell, who developed telepathic powers. In 2266, Captain Kirk and the USS Enterprise took aboard a passenger, Charles Evans. Subsequent events revealed that Evans had been granted various psionic skills by the enigmatic Thasians, so that he might survive on their world.
Telepathic races such as the Betazoids usually develop a moral code (sometimes making it actual law) that precisely dictates under what circumstances the powers may be used. Telepathy, misused, can be the ultimate invasion of privacy, and for that reason, it is feared by some.
Some telepaths can send and receive feelings and emotions. Sometimes this is part of a telepathic skill, and sometimes it is their only skill. In the latter case, individuals are referred to as empaths. A notable empath is Deanna Troi, daughter of a Betazoid and a human; she inherited a limited form of her mother's telepathic power.
A few civilizations have developed mechanical telepathy. Landru, a gifted engineer and philosopher, constructed a computer able to telepathically control most of the population of the planet Beta III. The engimatic Flint built a series of robots that could be controlled telepathically, and that could respond to mental emanations consistent with distress or danger. Sargon, Thalassa, and Henoch, the last survivors of an extremely advanced civilization, knew how to build robots their minds could inhabit and control.