The term snapshot aesthetic refers to a trend within fine art photography in the USA from around 1963. The style typically features apparently banal everyday subject matter and off-centered framing. Subject matter is often presented without apparent link from image-to-image and relying instead on juxtaposition and disjunction between individual photographs. This tendency was promoted by John Szarkowski, who was head of the photography department at the Museum of Modern Art from 1962 to 1991, and it became especially fashionable from the late 1970s until the mid-1980s. Notable practitioners include Garry Winogrand, Nan Goldin, Wolfgang Tillmans, Martin Parr, William Eggleston, and Terry Richardson. In contrast with photographers like W. Eugene Smith and Gordon Parks, these photographers aimed not "to reform life but to know it." (John Szarkowski, Diane Arbus) Szarkowski brought to prominence the work of Diane Arbus, Lee Friedlander and Garry Winogrand in his influential exhibition “New Documents” at the Museum of Modern Art in 1967, in which he identified a new trend in photography: pictures that seemed to have a casual, snapshot-like look and had subject matter that seemed strikingly ordinary.
Later photographers such as Hiromix, Ryan McGinley, Miko Lim, and Arnis Balcus gained international recognition thanks to the snapshot aesthetic. From the early nineties the style became the predominant mode in fashion photography, especially within youth fashion magazines such as The Face - photography from this era is often associated with the so-called 'heroin chic' look (a look often seen as having been influenced particularly by Nan Goldin)
The term arose from the fascination of artists with the 'classic' black & white vernacular snapshot, the characteristics of which were: 1) they were made with a hand-held camera on which the viewfinder could not easily 'see' the edges of the frame, unlike modern cheap digital cameras with electronic viewfinder, and so the subject had to be centred; and 2) they were made by ordinary people recording the ceremonies of their lives and the places that they lived and visited.
An early theorist of snapshot aesthetic was the Austrian architectural critic, Joseph August Lux, who in 1908 wrote a book called Künsterlische Kodakgeheimnisse (Artistic Secrets of the Kodak) in which he championed the use of Kodak cameras like the Brownie. Guided by a position that was influenced by the Catholic critique of modernity, he argued that the ease of use of the camera meant that people could photograph and document their surroundings and thus produce, what he hoped, was a type of stability in the ebb and flow of the modern world. 
- Mark Jarzombek. "Joseph Agust Lux: Theorizing Early Amateur Photography - in Search of a "Catholic Something"," Centropa 4/1 (January 2004), 80-87.