Short answers, pulled from the story.
Avant-garde is a French military term for an advance guard, originally describing a reconnaissance unit that scouted terrain ahead of the main army. By the mid-19th century the word had moved into French politics and art to describe innovators who challenged established forms. Henri de Saint-Simon's 1824 essay was among the first to apply it explicitly to artists.
Henri de Saint-Simon used the term in his 1824 essay "The Artist, the Scientist, and the Industrialist," arguing that artists had a moral obligation to "serve as the avant-garde" of the people because the arts offered the most immediate way to achieve social, political, and economic reform.
According to composer and musicologist Larry Sitsky, modernist composers such as Arnold Schoenberg, Elliott Carter, and György Ligeti do not qualify as avant-gardists because "their modernism was not conceived for the purpose of goading an audience." Peter Bürger defined avant-garde more broadly as work that rejects the institution of art itself and incorporates political and cultural factors.
In his 1939 essay "Avant-Garde and Kitsch," Greenberg argued that the artistic vanguard opposes both low culture and the artifice of high culture. He described mass-culture products in a capitalist society as kitsch, treating each mass-communication medium as a factory that produces simulations and simulacra of art rather than genuine art.
Arrière-garde is the rearguard counterpart to avant-garde. Art historians Natalie Adamson and Toby Norris argue it describes artists who engage with the avant-garde's legacy while knowing that doing so is anachronistic. Critic Charles Altieri stated it directly: "where there is an avant-garde, there must be an arrière-garde."
Avant-garde photography developed through Bauhaus-linked experimentation, cameraless techniques such as the photogram, and radical framing. László Moholy-Nagy was emblematic of Bauhaus experimentation; Man Ray produced cameraless "rayographs"; and in the Soviet context, Aleksandr Rodchenko used dramatic high and low angles to estrange familiar subjects. In Japan, an interwar movement including the Nagoya Photo Avant-Garde was shaped by contact with Surrealism, and the 1937 exhibition Kaigai Chogenjitsushugi Sakuhinten circulated Surrealist imagery through prints and photographic reproductions.