In a discreet basement in London’s Mayfair, the former art dealer Anthony d’Offay - best known for representing stars of international contemporary art from Andy Warhol to Rachel Whiteread - this summer opened a gallery devoted to the humble picture postcard.

In the popular imagination, the postcard is a brashly colourful image of a historic site or archetypal scene - Niagara Falls, the Coliseum in Rome, the changing of the guard or a palm-fringed beach. The survival of postcards today - in the age of e-mail and text-messaging, of instant digital picture-making and equally instant transmission - might be seen as a curious anachronism but, according to d’Offay, more people than we might imagine have a real passion for them. “It is often a secret passion,” he says.

D’Offay himself is one of them, and now, five years after announcing his retirement and closing his art galleries, he has opened the Postcard Archive in the basement of his old premises, fitted out with custom-made glass cabinets to display a rolling programme of exhibitions drawn mainly, but not exclusively, from his own vast collection.

The urge to exhibit is, he admits, “in my blood”, but d’Offay also sees his gallery as educational, and he emphasises the documentary importance of many of the little-known images caught on camera. “They’re virtually all unpublished,” he says. “It’s not just a chapter in the history of photography, but a huge slice of social history, and it is largely unknown.”

Only recently has anyone attempted to situate postcards within a critical history of photography. Photographer Martin Parr has drawn attention to the postcard’s mix of documentary content, precise pictorial codes and enigmatic appeal as a branch of popular art. Selections from his collections, published in books ironically titled Boring Postcards, demonstrate just how revealing the unsophisticated and seemingly mundane image can be to historians.

Parr’s personal interest in colour photography has led him to concentrate his attention on the postwar era, whereas d’Offay has focused principally on “real photo” postcards from what he calls the golden age - 1900 to 1925. He started to collect systematically about 10 years ago, initially for research connected to another collection of his own, of African head-dresses. “I wanted to find documentary material about them and started searching out postcard illustrations, because I knew there were a lot of things you could only find in postcards. The way these objects look in a London house is very different from how they looked sitting on people’s heads in the bush 75 or 100 years ago.”

D’Offay’s interests widened, and eventually he set out to construct through postcards a comprehensive visual history of the world in the early decades of the 20th century. He has long since lost count of the number of postcards in his collection, saying it is “probably between 10,000 and 15,000”, most of them acquired through collectors’ fairs or specialist journals such as Picture Postcard Monthly.

When they first became widespread in the late 1890s, the majority of postcards were actual photographs. Only later were they consistently printed on mechanical colour presses. What distinguished them as postcards was their near-uniform size and the crucial fact that they were printed on the reverse with guide lines for an address - the very earliest cards allowed no space for a message.

Technical developments in photography, notably new fast-exposure emulsions and easy-to-use hand-held cameras, put postcard picture-making within the reach of many and allowed a considerable range of subject-matter, including the instantaneous. Meanwhile, photographic postcard paper was manufactured on an industrial scale and was commercially available to cottage-industry postcard makers and distributors, for use by professionals and amateurs alike. In this way postcards became a kind of visual news service, cementing shared values and interests, underscoring a sense of social identity and community, much as television was to do half a century later. They were collected from the beginning, either directly or through postal exchange, and often displayed in specially manufactured albums.

The popularity of these cards owed much to the speed and reliability of postal services, and the absence of any alternative medium of communication that effectively combined photograph and text. The manuscript component, never intended for anyone beyond the recipient, can be a key aspect of a card’s documentary interest today.

“The postcard is the only form of photography that asks you to comment,” says d’Offay. “The message transports you to that place and time in a real way.” The message on the back of a postcard showing a native American girl at a parade, for instance, says much about the attitudes of the era: “This is the Indian princess, quite a pretty girl, but my how they all do smell,” it reads.

The Postcard Archive’s opening display brings together a range of cards from France, Germany, America, Japan and Russia. The Russian examples throw particular light on life before the revolution, with a flood of imagery released with the easing of censorship in 1905 - much of it criticising the tsar, the government and the way the country was run, from both the left and the right politically. Some of the cards reveal the hatred with which Rasputin was seen - incidentally supporting Lenin’s claim that without Rasputin there would have been no revolution.

The gallery’s first themed show this autumn, The American Dream, examines the social history of the US in the same period. Many images show the harsh realities of poverty and racism, and they include vicious trophy pictures of lynchings exchanged between gloating perpetrators, and portrayals of black children with denigrating inscriptions. Contrasts are extreme: a tattooed woman and a line-up of Ku Klux Klansmen; an electric chair and children in their Sunday best. The cumulative effect is highly emotive - a sweeping and disarmingly direct view of the ambitions, vanities, occasional joys and tough challenges of life for the generally voiceless masses.

This is photography as witness and document; we can feel the dust, the cold, the hunger, the pride and the anxiety. The pictures are direct, naive and, although often well composed and with occasional charming folk-art interventions, they are untouched by grand artistic debates. These postcards are the visual equivalent of popular song - campfire music or Delta blues - showing us America in the raw, at once harsh and tender, full of grit, poignancy, delight, brutality and pity.

“The American Dream” opens on October 12 at Postcard Archive, 9 Dering Street, London W1.