A positron is the antimatter version of an electron. It has the same mass and spin as an electron, however it has a positive charge, whereas an electron has a negative charge. Like all antimatter, when it meets its so-called counterpartner, each are annihilated and turned into energy. The electron and positron disappear, and the total mass decreases. There is no name for this type of energy, as it is neither mechanical, radiation, chemical, electrical, nuclear, nor thermal. Although the energy does emit light photons when annihilated, it is converted from some form of energy that has not been named.
A positron also very rarely makes a structure called positronium. Positronium is like an atom in many ways, but is very unstable, and usually quickly annihilates anyway.