Antimatter is a rare, separate type of matter formed of particles with some of their quantum numbers the opposite of those in corresponding normal matter, among these differing quantum numbers being electrical charge. When a normal matter particle encounters an antimatter particle, they annihilate each other, the energy converted to photons. Antimatter is extremely rare locally and it is assumed that the observed universe is basically normal matter. There is no consensus as to the cause of antimatter's rarity.
Antimatter particles include the Positron (i.e., Antielectron, like an electron but positively charged), the Antiproton, the Antineutron, Antineutrinos, and Antiquarks. The particles can be combined to form "anti atoms", e.g., Antihydrogen, and this has been done experimentally.
In minute quantities, positrons and antineutrinos are commonly produced by Beta Decay (thus in Nucleosynthesis) and other particles are commonly formed by high-energy collisions, e.g., in particle accelerators, and by Cosmic Rays striking ordinary matter. The particles are short-lived when surrounded by normal matter.