Why outlets spark when unplugging - EMF & Inductors

what is the difference between an arc and a spark because they both behave differently?
also when a spark is created it has a blue colour is this because of the nitrogen in the air gives this spectrum of light? and if so then why do they say there is ultra violet like a TEA laser makes,well if its uv then what about the blue light why is this not part of the laser beam?