--- A plasma is a gas containing free electrons and positive ions. It conducts electricity.
--- Sun, magnetosphere, ionosphere and solar wind are all plasmas. So is the gas inside a fluorescent tube.
--- A few free ions and electrons are always around--due to radioactive traces, etc. The voltage on the tube accelerates them, they collide and knock off more electrons in the gas. The plasma quickly gets much denser.
--- The ballast coil stops the plasma from getting too dense, and the current too big.
--- Electrons and ions recombine all the time, and produce light. The fluorescent paint on the inside converts useless ultra-violet to visible light.