Hard sci-fi plasma weapons come in types: “flamethrowers” or cannons that expel stored plasma, Fusion cannons, melta weapons where so much energy is expelled from a weapon that it turns the air outside into plasma, lightning guns such as Tesla coils, with a large pulse of energy sending a huge electrical charge that produces plasma conduits across the air, or Ion Cannon like the ones that GDI uses as its signature superweapon. In fantasy sci-fi, “plasma” weapons fire insubstantial ammunition that imparts extreme heat on impact, or just as with gauss weapons, plasma guns can be anything that sounds cool. The tokamak, the most promising candidate for a working fusion reactor, exploits this by accelerating plasma in an intense electromagnetic torus until it becomes so pressurized, hot and fast enough that fusion is achieved, although the cost of power outweighs the possible electrical production benefits in the current generation of the technology. The plasma is then contained within an electromagnetic field, keeping it out of contact with the weapon’s components which it would almost certainly damage. Plasma is produced by applying a very strong electromagnetic field to a gas, such as hydrogen. Nuclear fusion requires hydrogen gas to be heated to the plasma state to overcome the Coulomb barrier between nuclei. Plasma is the most common state of (regular) matter in the universe this is because intergalactic space consists mostly of low-density plasma, and stars are giant balls of high-density plasma. Seeing as it is a mass of charged particles, it is also highly susceptible to influence from electromagnetic fields.
This means plasma is incredibly hot and incandescent, though unless it is optically dense (thick enough to reabsorb the vast majority of emitted photons), it cools rapidly.
Plasma emits truly heroic amounts of energy, both as heat and light, because the free electrons within the mass are frequently caught in the attractive pull of the nuclei, resulting in circular motion around the nuclei, which decelerates the electron, causing it to emit a photon in order to maintain conservation of energy (Bremsstrahlung emission).
It is a sea of ions, or charged particles exposed to so much energy, such as nuclear fusion or lightning, that the bind between electrons and nuclei become weakened and the electrons freely flow through the plasma as a current just like through a metal. Plasma is the fourth state of matter, beyond gas. “Never was anything great achieved without danger.”