The Powers of the Mind
Psychic powers are often mysterious, even to the ones that have them, and it takes a great deal of practice and discipline before the abilities can be used consciously at their full power. Most psychics manifest their gifts when they are young. As they age the power waxes with them with most psychics reaching the peak of their powers in adolescence, as the storm of hormones helps pump up their psychic powers prior to settling into a 'stable' configuration into adulthood and beyond.
Psychic abilities are sometimes referred to as Psionics, a term which was developed and cultivated by scientists seeking to understand how psychic abilities are generated, and how the powers manifest. Few psychics use this term when speaking of themselves, though there are a few academics with psychic abilities that may use the term.

Psychics are generally reluctant to let their powers and abilities be known. Because the abilities seem to originate with and are governed by the mind alone, they are often the subject of less than ethical scientific experiments by those desperate to understand the powers used by these seemingly normal people.
Where do psychic powers come from?
There is no clear origin for psychic abilities, although some scientists theorize that there is a gene which controls psychic abilities and that these abilities can be transmissible into family lines. Others view psychics as mutations whose abilities are the result of exposure to solar energy or the conflux of rare and unusual planetary alignments which put out increased radiation of some sort.
A less secular theory suggests that psychics are the literal descendents of the Nephilim which are referenced in the Book of Genesis and in greater detail in the Book of Enoch. Believers of this hold that while the Nephilim may have been destroyed by other angels, the bloodlines continued onward and that eventually these bloodlines join, creating an offspring with psychic powers.
What is known is that people from all around the world, and through all ages of the world have exhibited what are today called psychic powers.
The Link to Emotion
Young psychics often generate or exhibit their powers spontaneously, during temper tantrums or other periods of heightened emotional stress. This link remains throughout the rest of their lives and so psychics, especially ones with destructive capacity, must learn to control their emotions. Some manage to do this easily while others are often on the edge of some metaphorical cliff. When psychics lose control of their emotions it is possible for them to lose control of their psychic abilities as well and to use them reflexively.

Demon-Blooded Psychics
Only known to a very few souls, some powerful demons are able to instill their blood with the power to grant psychic abilities. Some demons do this out of sheer joy and others because they desire foot soldiers or aids in one clever scheme or another. In either case, demon-blooded psychics tend to have more numerous and more powerful gifts in addition to powers that are exclusive to demon-blooded psychics.
Read more about demon-blooded psychics here.