In a clearly developed and obvious form, these senses are generally quite uncommon, though it is suspected that most (if not all) creatures do possess them to a very limited degree; this theory is furthered by the variety of species types that show the capability and that it is not unique to sentient beings.
Telepathy is the ability to read the thoughts of others. The range and how strong (often stated as “loudness”) the thoughts are play an important role in whether or not they can be “read”. No telepathic race has ever shown any capacity to broadcast their thoughts into the minds of others who were not themselves telepathic, and none are able to tap into the minds of others to gain insight into memories and other information not actively being processed by the conscious mind. The phenomenon is fairly well understood to be a trait of certain nerve receivers which then carry the information to one or more specialised brain lobes which then interpret the information. These sensory nerves are sensitive to the detectable emanations of certain parts of the cerebral cortex (or its various analogues across the diversity of species).
A common myth about telepathy beyond the ability to place thoughts or to mind-probe is that it means that all thoughts can be understood. This is quite untrue. Any conscious reminiscence that occurs can be “seen” in the same way that the person remembering is “seeing” the recollected images. The language that the person is thinking in is the one “heard” by the telepath. No special understanding is gained. Also the more alien the mind the less likely that the telepath will receive anything or that she will be capable of understanding it even if the thoughts are in a comprehended language.
Empathy, on the surface, seems to be a linked trait to telepathy, though they do function on rather different principles on the biological level. The ability to pick up and interpret emotions is derived from specialised tissues which read a number of things from brain impulses to pheromones and this information is carried to the brain for analysis in a structure that is usually located much closer to the mid or hindbrain than to the cerebrum (again, or analagous structures).
Ability to sense emotional states does not, of course, carry with it any understanding of the causes. Contrary to popular holo-serial characters’ depictions, for example, one can tell that a person is feeling love, hate, jealousy, etc. but must rely on observation or questions to determine for whom the emotion is felt. Similarly a telepath cannot interpret emotional states, if the person’s thoughts provide no clues then she is no more able to tell how a person feels than could someone who can hear or see the individual.
Some technologies do exist which are designed around the understanding of telepathic senses, and even some empathic variations of them. Magvinnians, for example, have designed most of the interactions with their computer systems to work on a computerised version of the their own telepathic senses as well as ways to record and play back thought and emotional patterns in a fashion not at all unlike recording and playback of sound, which allowed them to retain their heavily neural based language in long range communication as well as the development of entire entertainment media such as thought and feeling symphonies.