Héliogravure is a form of intaglio printing.  Like etchings and engravings, the print starts with a plate, and, like etchings and engravings, héliogravure plates are typically copper.  Unlike etchings and engravings, the image on a héliogravure plate is not mechanically created.  Instead, it is created with a photochemical process.

To prepare the plate for the photochemical process, it is dusted with rosin powder.  (Rosin is the solidified resin obtained from pine trees.)  The plate is then heated so that the grains of rosin melt into fine droplets and fuse to the copper plate.  The rosin dust particles are microscopic in size.  Therefore, when they melt, they create an extraordinarily fine grid, leaving miniscule gaps for the acid to pass through to the plate.  The result:  An exceptional tonal range, with precise transitions from light to dark and the ability to obtain nuanced changes in color and texture.

Like etchings and engravings, the héliogravure plate is then placed on a press and printed onto dampened etching paper using special inks.

How do you know when a print is a héliogravure?  Examine the print under a magnifying glass.  Unlike offset prints, rotogravure, and even letterpress, you will see no screen pattern in the image.