Realism painted ordinary life exactly as it looked, in dark, precise detail, finished in the studio. Impressionism painted the same ordinary life fast, outdoors, in bright broken color, chasing light rather than detail. Impressionism grew directly out of Realism, about twenty years later.
One movement cleared the ground. The other built on it.
Realism came first
Realism took shape in France around 1848, led by Gustave Courbet, who painted peasants and workers at the same scale once reserved for kings and saints.
His 1850 painting A Burial at Ornans showed an ordinary funeral in a small town, at a size normally kept for history painting. Critics found the subject beneath the format.
What each one was after
Realist painters wanted the world shown without flattery: dark palettes, careful detail, and social subjects, often the poor and the working class.
Impressionist painters kept the everyday subjects but dropped the social commentary. They wanted the light and the moment, not a message.
How the paint differs
Realist paintings were built in the studio, layer by layer, with smooth transitions and visible skill in the finish.
Impressionist paintings were mostly built outdoors, fast, in short strokes of unmixed color, often finished in a single sitting.
I think Realism's real provocation was scale, not subject. Painting peasants life size was what upset people, not the mud on their boots.
The same fight, twice
Courbet had already fought the Salon in 1855, setting up his own pavilion outside the official exhibition after being rejected.
The Impressionists organized their first independent show in the same spirit, nineteen years later, in 1874.
See where that show led in Famous Impressionist Paintings.