There are numerous critical approaches to literature, some more illuminating than others. A number of these approaches can be put under the broad heading of formalism, which encompasses New Criticism, which was dominant in the mid-twentieth century, and Russian Formalism, which enjoyed a brief vogue in the early decades of that century.
Although there are clear differences between the various formalist critical schools, they share a number of important similarities. The most significant is a focus on the formal aspects of a literary text.
In a novel, this would include plot, characterization, and structure, not to mention diction and style. In a poem, the focus would be on formal elements such as rhyme, rhythm, and cadence. Certain poetic techniques such as alliteration, consonance, and assonance would also be closely examined as part of an effort to get at the meaning of the text.
One could argue that this is the best critical approach to literature because it concentrates on what really matters, which is the text. Present-day critical approaches to literature tend to incorporate all kinds of elements extraneous to the text, such as biographical details, political ideology, and the historical conditions under which the relevant text was written. By contrast, formalism gets to the heart of the matter by concentrating on what's right there in the text.
That doesn't mean, of course, that formalist critics completely exclude what isn't in the text in their evaluations. But it does mean that in prioritizing the text and its formal features, formalists don't lose sight, as all too many present-day literary critics do, of what's most important in examining a work of literature.