Platinum, palladium and platinum-palladium pritning is high-grade technique of photography printing found in 1870 which has been replaced as main printing method by silver processes based on silver. Comparing with enlarging on paper with silver base where the picture is created by cristals of silver the picture here is created by noble metals platinum and palladium. That is why it is one of the most expensive technique of photography printing. 
Due to the fact that I am not a chemician I will not describe chemical reaction which leads to creation of the picture. Nevertheless, although platinum and palladium print can not create such a dense black as silver processes, platinum and palladium print is unique by its rich scale of midtones. Besides that one of the biggest benefits of these processes is longevity of the prints. Well done print can last substantially longer than photography on silver. Some say that 1000 years which can hardly be verified but the fact is that more than 100 years old platinum prints have been preserved until now probably without substantial change from the date of creation. 
Exposure is not made by enlarger but by UV light source and with much more time of exposure than processes based on silver. This is one of the reasons that a print is made by contact of negative with sensitive layer on paper. I use UV exposure box made by myself with UV tubes 4 x 40W.  
I use solution with 1/3 of platinum and 2/3 of palladium for printing. Chemicals are from Bostick & Sullivan. Prepared solution is hand coated at paper especially determined for platinum printing - Arches Platine in my case. I use Hake Brush from goat hair for coating. Some printers use glass rod for coating which I may try sometimes.   
Maybe once I will get to making complete process by classic way, however, I make negatives by inkjet printing at transparent foil Pictorico Pro OHP now. 

Gallery of platinum-palladium prints scans