In the Catholic tradition, holy cards or prayer cards are small, devotional pictures mass-produced for the use of the faithful. They typically depict a religious scene or a saint in an image about the size of a playing card. The reverse typically contains a prayer, some of which promise an indulgence for its recitation. The circulation of these cards is an important part of the visual folk culture of Roman Catholics.
The invention of colour lithography (also called chromolithography) made it possible to reproduce coloured images cheaply, leading to a much broader circulation of the cards. An early centre of their manufacture was in the environs of the Church of St Sulpice in Paris; the lithographed images made there were done in delicate pastel colours, and proved extremely influential on later designs. Belgium and Germany also became centres of the manufacture of holy cards, as did Italy in the twentieth century. Catholic printing houses (such as Maison de la Bonne Presse in France and Ars Sacra in Germany) produced large numbers of cards, and often a single design was printed by different companies in different countries.
A lot of them are really cool and strange.