The Carte de Visite format, or CDV, were Albumen (egg white) based prints in a small, convenient to mail format measuring 2.5" x 4". Most of them are of individuals or couples, and either portraits, ie: head & shoulders, or full figure.
 

History:

In 1854, Andre Disderi, a Frenchman. patented a process to create 8 small negatives on a single 8" x 10" plate as well as the Carte de Visite format itself.. CDVs were first offered in Europe in 1859 and in the US in 1860, and were an instant hit. They allowed Civil War soldiers to send photos to their friends and families as  they overcame the fragility and thickness of the glass Ambrotypes that they replaced.

Carte de Visite translates to 'Visiting Card', a carryover from the custom of leaving a calling card upon visiting friends and acquaintances. Then as now, people loved to collect them, and millions were produced in the US. They were even more popular in England, where it is believed hundreds of millions were produced each year. The craze was given the name 'cardomania'. With the popularity of the Carte de Visite came a new business opportunity: producing and selling celebrity CDVs. CDVs of celebrities, authors, war heroes, famous criminals and other notables were avidly collected, including President Lincoln and his assassin, John Wilkes Booth, Ulysses S. Grant, etc. Celebrities began to ask large sums (up to hundreds of dollars in those days) for the right to market their likenesses. Counterfeits became commonplace, though these often lacked the signature backmark of the  authorized versions.
 

Backmarks:

The earliest CDVs generally had simple typed photographer attributions. These rapidly became more elaborate backmarks advertising the photographer. They were anxious to be perceived as true artists and often described themselves using that term. The backmarks are helpful in genealogy, as they usually identify the city and state where they were taken. Many of the backmarks were works of art in their own right, and some collectors specialize in backmarks. 
 

Post Mortems

Death was an everyday fact in the Civil War and Victorian eras.  Between high infant mortality rates, Civil War, and the limits of the medical practitioners of the day, loss of loved ones was all too common. There were specific protocols for the mourning process; for example, a widow would wear black for 1 to 2.5 years after the death of a husband, would not remarry for at least a year, etc. There were different rules depending on the immediacy of the relative, and for children under 10 years old. Photographing the deceased was a way to keep them near, and as photography was still a novelty, burials often had to be postponed until the photographer could travel to the location. In the case of infants, this was often the only photograph of the child. Post mortem photographs are actively collected, as are memorial or death cards (which were given out at memorial services and sometimes featured a photo of the deceased in life), and photos of those in mourning  dress.  
 

Outdoor Carte de Visites

Due to the bulky equipment, most photography of this era was done in the studio. Outdoor CDVs are considered very desireable. There were a number of photographers who traveled and set up at fairs and other events and others who traveled the new frontiers, camera in wagon. They would capture landscapes, the daily life of the time, town and events, as well as people at work, known as 'occupationals'.
 

Occupationals

Another area of specialization is the collection of occupationals, or people in their work life. These include such subjects as carpenters with their woodworking tools, doctors with medical equipment, bakers with their wares, or any of a myriad of subjects. While books are often featured in peoples hands or on side tables (to indicate education and literacy), they are sometimes identifiable as related to the occupation of the subject, a very desireable thing. Similarly, some collect photos of children with interesting dolls or toys.

By the early 1870s, the Cabinet Card format was becoming the dominant format, as they could be displayed in or on cabinets and still be visible across a room.