The field gives the force a unit charged particle would feel when placed in the field of another charged particle. The field exists at every point in spacetime (for long range fields). I always think of a field as being like the disturbance created by throwing a rock into a pond, which propagates through the water. But if the field exists everywhere in spacetime, doesnt this mean for a classical field a static charge is constantly and forever losing energy in the form of the field? Going back to the analogy of the water waves, to create a field at every point in spacetime, one would have to have something osciallting for eternity in the water creating new disturbances, which would take an infinite amount of energy. How can this be? In the quantum theory, the particle undergoes quantum fluctuations due to the uncertainty principle. Could this infinite vacuum energy be the source of the infinite energy of the field?