This lecture introduces the concept of plasma state, a complex medium of ions and electrons responding to electric and magnetic fields. It covers the challenges of understanding plasma behavior, the basic phenomena in plasmas, and the models used to describe plasma dynamics. The instructor, professor Paolo Ricci, will provide a rigorous definition of plasma, discuss important spatial-temporal scales, and evaluate the trajectory of charged particles in electromagnetic fields, shedding light on plasma confinement principles for fusion energy applications.