Bakhtin argued that early Formalist and Structuralist critics did not articulate the "sociological stylistics" of literature. These critics focused too much on the unitary, abstract rules of language and the common literary elements that give a text its literariness. Bakhtin recognized the dynamics of the novel. The novel, more than other forms of literature, incorporates living language; not a series of monologues and/or dialogues under the control of one single poet. Therefore, Bakhtin studied the novel's multiplicity of voices; it's "heteroglossia."
Literally meaning "different languages," heteroglossia is the term Bakhtin uses to describe the different and interrelated social, class, and generational dialects in the novel. These different dialects, via characters and narrators, speak to each other. Bakhtin says that this is true of all language but in literature, it is most accurately expressed in the novel. Therefore, he says that novels tend to be dialogic while poetry tends to be monologic. Bakhtin's concept of dialogics was a precursor to Kristeva's use of "intertextuality" where all languages and all texts are essentially in a dialog with each other. So, looking at a novel, there is not one voice dominating its discourse; there are many voices of differing class, politics, etc. Here is one excerpt from Discourse in the Novel where Bakhtin elaborates on dialogics, particularly in the novel (prose):
The prose artist elevates the social heteroglossia surrounding objects into an image that has finished contours, an image completely shot through with dialogized overtones.
To state an obvious example, look at one of Browning's dramatic monologues, "My Last Duchess," in which there is literally one speaker. He is addressing another person but it is hardly a dialogue. Even in poems where there are different characters, such as Pope's "The Rape of the Lock," Bakhtin would say that the poem as a whole is guided by one voice, one social perspective. In Discourse in the Novel, Bakhtin cites Tolstoy as exemplifying the dialogics and heteroglossia of language.
". . . Tolstoy's discourse harmonizes and disharmonizes (more often disharmonizes) with various aspects of the heteroglot socio-verbal consciousness ensnaring the object . . . "
In other words, heteroglossia is the range of interacting social dialects, class positions, political ideologies, and ways of speaking. In this dialogical form, these differing voices speak to each other, and to other historical voices. In a general sense, heteroglossia is how all dialects, how all words really, refer to one another, thus recognizing the real living language that happens in real social interactions. Bakhtin thought that the novel was the best literary form to present this complex interaction.