First, a definition:

Cyanotype (n): /ˈsīənəˌtīp, sīˈanə-/
A photographic blueprint. (oxforddictionaries.com)

The process (in brief):

Paper (or other material like fabric) is painted with a solution of potassium ferricyanide and  ferric ammonium citrate in a dark room.  The paper is covered with a contact print or object and exposed to ultraviolet light.  The paper is then washed in water;  the parts exposed to the light turn blue and the parts blocked by the image on the print or the objects remain white.  Depending on the length/strength of exposure, there could be a wide range of values.  This process was used to make traditional blueprints.

Some History:

This process was discovered in 1842 by Sir John Herschel, and was mainly used to reproduce notes and diagrams.  Anna Atkins used this for photography creating cyanotype books documenting plant life.  She is known as the first female photographer. 

Anna Atkins