Remote Sensing: 

Remote sensing is an art and science of obtaining information about an object without physical contact with that object.Imaging sensor systems are of two types: Framing systems and Scanning systems. In Framing system images of the targets are taken frame by frame. These include imagers like photographic film cameras and return beam vide  A Scanning system employs a single detector with a narrow field of view that is swept across the terrain to produce an image. 

Electromagnetic spectrum is not a Remote sensing system rather it is the distribution of the continuum of energy plotted as a function of wavelength or frequency.