Short answers, pulled from the story.
Realism in art is the attempt to represent subject-matter truthfully, without artificiality, exaggeration, or speculative or supernatural elements. The term is often used interchangeably with naturalism, although the two are not necessarily synonymous.
The Realist movement began in the mid-19th century as a reaction against Romanticism and History painting, originating in France in the aftermath of the French Revolution of 1848. Its chief exponents were Gustave Courbet, Jean-François Millet, Honoré Daumier and Jean-Baptiste-Camille Corot.
"Realist" art concerns subject matter, while "realistic" usually refers to accurate visual appearance. A realist painter like Gustave Courbet was not especially noted for fully precise depiction of visual appearances, which in his time was more often a feature of academic painting.
Naturalism is the precise, detailed and accurate representation of the appearance of scenes and objects, also called mimesis or illusionism. In 19th-century Europe it was erected as a breakaway sub-movement of realism that avoided politics and claimed a quasi-scientific basis, with the term coined by French critic Jules-Antoine Castagnary in 1863.
Realism spread across the arts as literary realism, theatrical realism, the opera style of verismo, and Italian neorealist cinema. Émile Zola carried naturalism into the novel, verismo brought gritty lower-class drama to opera through composers like Pietro Mascagni and Giacomo Puccini, and Italian Neorealism developed in post-WWII Italy with directors including Vittorio De Sica and Roberto Rossellini.
Realist painters used common laborers and ordinary people in ordinary surroundings as subjects, building on a long tradition of depicting everyday life. Early Netherlandish painting brought portraiture down to the prosperous merchants of Flanders, and 16th-century artists such as Pieter Aertsen and Pieter Bruegel the Elder painted markets, kitchens and panoramic scenes of peasant life.