structures or by arbitrary rules. These conventions developed out of specific circumstances, through dialogic communication, which is always a negotiated process.

Bakhtin believed that language should be studied for its dynamic, real life, elements of speech and social relations. He did not think, as some structuralists did, that language is an abstract system because such thinking ignored language as a living, subjective, system of fluxing social relations. 

Bakhtin used the term "heteroglossia" ("hetero" meaning "different" and glossa meaning languages) to describe how language is an ever changing complex of social dialects, class dialects, jargon, fads, and intertextual references. Bakhtin concluded that the novel is the best genre to showcase the heteroglossia of language because the novel includes more elements of everyday speech and thereby, more dialects, and a greater range of social, class, and cultural languages. Bakhtin believed that poetry can showcase heteroglossia but that its limitations to poetic diction limit its emphasis on different languages and different voices. Therefore, Bakhtin saw poetry as largely monologic and the novel as dialogic. In other words, he believed that the novel is the literary genre which had the most elements of these different manifestations of language: class, social, interpersonal, and political conflicts and relations. 

Bakhtin looked at how language operates with and through politics, ideology, class conflicts, intertextuality, and historical contexts. In other words, every utterance, once spoken, does not stand alone; it becomes involved in a dialogue (dialogic) with other utterances: 

The living utterance, having taken meaning and shape at a particular historical moment in a socially specific environment, cannot fail to brush up against thousands of living dialogic threads, woven by socio-ideological consciousness around the given object of utterance; it cannot fail to become an active participant in social dialogue. (From Discourse in the Novel) 

Just as there are different class, societies, and cultures within a given country or state (who use the same language, English for example), that dialogue amongst those people is also stratified with those different overtones that come from different classes, social groups, and cultural backgrounds. Bakhtin argues that with poetry, we mostly hear the poet speak: a monologic voice. But in the novel, there is more opportunity to hear the characters speak. And in the novel, we have more chance to hear different voices ("languages") from doctors and lawyers to the poor to politicians and aristocrats. 

Even a single word itself is not simply the signifier of a signified meaning. That word/meaning will mean something different because of who said it, the historical context in which it is used, and so on. Bakhtin studied language in its living form, as a dynamic interplay, acknowledging historical context and social differences. 

Bakhtin believed that the basic units of language, such as words or utterances, do not have abstract meanings but are instead affected by the speaker's relationships with other people and the culture of the time. Therefore, Bakhtin concerns himself with the cultural and historical context of language as well as what Bakhtin calls "addressivity" (to whom language is addressed) and "answerability" (the response that language invites). Therefore, language never exists in a vacuum but is affected by its historical context and the dialogue in which it is involved. Bakhtin writes in Problems of Dostoevsky's Poetics, "I live in a world of others' words." In other words, language is not neutral but is affected by other people's meanings. He writes in Speech Genres and Other Late Essays:

"When we select words in the process of constructing an utterance, we by no means always take them from the system of language in their neutral, dictionary form. We usually take them from other utterances, and mainly from utterances that are kindred to ours in genre, that is, in theme, composition, or style. "

As utterances exist in a world of other people's words, utterances in any genre are marked by "heteroglossia," or multiple voices and styles within one work, and the work is "polyphonic," or characterized by many voices. Therefore, Bakhtin is concerned with the way in which language calls upon other people's meanings and operates within a deep cultural and historical context. 

Bakhtin was concerned with two main aspects of language: linguistics and literary theory. As my answer to your other question suggests, the two were intimately related in his mind. Basically, Bakhtin was interested in the "dialogical" aspect of language. This means that he thought language involved reaching mutually agreed-upon meanings, not necessarily referencing meanings that already existed. Meanings are always changing according to a number of different factors. But communication depends on being able to reach mutually agreed upon meanings, so language, while always relational, was never completely relativistic. People have an interest in being understood, and in understanding, so they reach agreements as to meaning.

Bakhtin's ideas around language concern the way that he saw humans as being part of a community and his view that language meant they could never be viewed as being isolated from each other. Note how he expressed this view:

In order to understand, it is immensely important for the person who understands to be located outside the object of his or her creative understanding--in time, in space, in culture. For one cannot even really see one's own exterior and comprehend it as a whole, and no mirrors or photographs can help; our real exterior can be seen and understood only by other people, because they are located outside us in space, and because they are others.

Bakhtin therefore placed immense importance on the role of others in understanding the voice of any one given person. Without this network that all humans are a part of, Bakhtin argued, it is impossible to communicate. Language is not truly an individual matter, but for Bakhtin, is something that is based on relations between individuals.

For Bakhtin, what he called the "utterance" is the basis of his theory of language. This utterance is not a statement in itself but it is importance because it represents the intersection of various contributing voices. Language is therefore never static in its presentation; it is always by its very nature something that is constantly fluid and changing, as there are any number of "utterances" that can be said at any given point. Language, as conceived by Bakhtin, becomes an exciting demonstration of spontaneity and improvisation as plural voices make meaning through their intersections with each other.