In a nut shell, cyanotypes are the original “blue print” and also known as “sun prints”. An iron-compound emulsion is painted on to a porous surface (i.e. paper), which when exposed to UV light turns a rich Prussian blue. If anything blocks the UV rays from hitting the paper this is ‘protected’, so that when you wash the emulsion out it remains white. You end up with striking blue and white prints. 

It is dubbed as 'alternative photography' in so much as it is not seen as a traditional photographic process - certainly nowadays where we rely heavily on digital cameras and digitally imagery.