Realism can be considered both a movement and a technique. The Realist movement first developed in France around the middle of the nineteenth century. Realism was popularized by authors such as Honoré de Balzac, Gustave Flaubert, Émile Zola, and Guy de Maupassant. Part of the Realist movement, though, was called Naturalism (as seen in Zola's writing--Germinal).
The Realists wanted to show life from an objective point-of-view. What this means is that they would "stand back" from their texts and offer unbiased descriptions of life as it was, without intervening on behalf of the protagonist. (Instead, they would "allow" the choices of the characters to fall without help.) One popular characteristic of the movement was the impact of nature on the people depicted in the works (seen through the personification of nature).
Outside of that, one could easily call it a technique based upon the fact that the authors used specific characteristics in their work. (I bring this up because an author could easily duplicate the Realist's way of writing by adhering to specific characteristics found in Realist texts.)
While modern texts could not be considered Realist, based upon the fact that the "movement" has "passed," one could very easily duplicate the power of nature, the use of the third-person omniscient narrator, the everyday settings, and the common working class protagonists and antagonists.