A photomechanical process of reproducing an image or design, it is basically a combination of photography and etching. Crucially, the gelatine in the photographic negative of the image acts as the acid resist when the image comes to be etched. There are two types of photogravure. Hand photogravure, hardly used now but very popular in the later 19th century, involves the photographic transfer of the image to a copper plate, prepared with aquatint to give it tone, into which the design is etched. The plate is then hand-inked and printed from in the same way as an ordinary intaglio plate. Machine photogravure, very much more commercial, is similar except that a cross-line screen, instead of aquatint, is used to provide the tone, and a cylinder is employed rather than a plate. Very fast printing in large editions, for example of magazines, is possible with this technique.
http://www.vam.ac.uk/vastatic/microsites/photography/processframe.php?processid=pr017 Description and illustrations on Victoria and Albert Museum website.