Telepathy is the communication of messages or thoughts between minds by some means other than sensory perception (vocal or auditory).
Touch telepathy is communication directly with another being's mind brought about by physical contact (Vulcan Mind Meld).
Some species, like the Cairn and Lumerians, can only exchange thoughts with other members of their species. Others can only receive thoughts, but not send them, like Betazoids, who can only exchange thoughts with other empaths/telepaths, but can receive the thoughts and emotions of most other species. Still others can only send, but not receive.
Person to person telepathy can be present in all heathly individuals of certain species. In others, like Humans, it occurs less frequently, but is possible (i.e. in rare circumstances individuals have gained telepathic powers). And in some, like Ferengi, it never occurs at all, which also makes it impossible for telepaths to read a Ferengi's mind.
Since some fear the misuse of telepathy (i.e. probing of someone's mind without consent) and regard it as the ultimate invasion of privacy, telepathic species, like Betazoids and Vulcans, have developed a moral code that dictates precise reasons and ways in which the power may be used.
- Memory Alpha, "Telepathy"