Russian formalist critics, who had begun to move toward formalism as a result of technical analysis of works like Pushkin's, adopted Saussure's linguistic work on signs and signifiers and the interplay of words in language. In other words, they incorporated the concept that the meaning and usage of any given word depend upon the meaning and usage of words adjacent to it resulting in both infinite possibilities and authorial style. To illustrate this, think of both Hemingway and Dickens writing that an interesting character is going to the grocery store--add Poe to the imaginary mix if you are still unclear on the idea of infinite possibilities due to both the breadth and restrictions of adjacent words. Thus, technical analysis plus Saussure's signs and signifiers formed the early foundation of Russian Formalism.
The concepts that were emphasized before the enforced split to Prague Formalism were, briefly, poetic/literary devices and structural forms. Rhyme, syntax, consonants, and plot were some of the more important poetic/literary devices that Russian Formalists emphasized within analyses that emphasized the important point that meaning is drawn from arrangement of words within the work itself and not from words associations with external sign referents. What the Russian Formalists rejected was the age-old Aristotelian notion that literary works carried relevant subject matter, significant philosophical ideas, and/or moral messages that should occupy the attention of a critic. In other words, the foundation of Russian Formalism was that literary works should be evaluated for verbal qualities alone.
When Marxism rejected the formalists and certain ones fled to exile, the Prague school of Formalism added concepts such as defamiliarization, the way in which devices are applied to words or elements so that the images and concepts are not seen as reflections of the world but as a way of making the commonplace "strange" in order to emphasize the meaningful interiority of the language of the work through a form of linguistic dislocation. For example, a waffle in a poem or story becomes a symbol of a complex patchwork of inter-related events whereas, in daily experience, a waffle is simply someone's favorite breakfast because it holds a lot of syrup.
Defamiliarization was further expanded by the Prague school to the concept of foregrounding, the way in which words and elements rise to the foreground from the background of the text to emphasize the interiority of the work. In other words, devices like diction choices, metaphor, ambiguity, parallelism, and theme foreground some elements while repressing others so that the interiority of the work is emphasized to result in a closed effect upon the reader.