I found the summation of heteroglossia, as well as its relationship to other key Bakhtinian terms, by Karine Zbinden in "Traducing Bakhtin and Missing Heteroglossia," in Dialogism 2, both succinct and clear. In this passage she is summarizing from "Discourse in the NoVel' (DN, pp.84-86 R; pp.96-98 f;pp.271-73 E).

... a professor ... insists that heteroglossia is roughly (entirely) synonymous with the post-structuralist concept of discourse. This article explains how such 'missreadings' can occur.

"Bakhtin begins his argument about centripetal and centrifugal forces with a few historical observations: he interprets the efforts towards the unification of the national languages in Europe, be they the various poetics, Leibniz's universal grammar or Humboldt's ideologism, as instances of centripetal forces. Bakhtin details the diverse ways in which other 'dialects' have been suppressed. He goes on to explain that centripetal and centrifugal forces are at work within a single natural language as well. In fact the situation within one natural language is comparable to and can be represented by the fight between the various 'dialects' or languages in a polylingual society. Thus as single natural language is not only stratified into dialects proper but into 'social-ideological' languages. This heretogeneity of natural language is heteroglossia. Heteroglossia thus accounts for both the common social nature of language as a shared code and for the individual appropriation of language in use. The notion of speech genre as a further development of the concept of heteroglossia introduces the idea of stability in language. This prevents the post-structuralist drift of a subject which speaks itself into self-annihilation, which is the back-cloth of an intertextual conception of language" (51).

Pam Morris's "The Bakhtin Reader" (1994) provides a wonderful glossary of terms (pp. 245-52). Here's Morris's definition of "heteroglossia":

For Bakhtin, discourse always articulates a particular view of the world. According to Bakhtin, earliest societies were characterized by "monoglossia," or a stable, unified language. "Polyglossia" refers to the simultaneity of two or more national langauges in the same society, a phenomenon which developed, as Bakhtin points out, in ancient Rome and during the Renaissance. "Heteroglossia" (the Russian "raznorechie" literally means "different-speech-ness"), refers to the conflict between "centripetal" and "centrifugal," "official" and "unofficial" discourses within the same national language. "Heteroglossia" is also present, however at the (q.v.) mirco-linguistic scale; every utterance contains within it the trace of other utterances, both in the past and in the future. The discursive site in which the conflict between different voices is at its most concentrated is the modern novel (q.v.). One way of representing heteroglossia in the novel is by a hybrid construction, which contains within it the trace of two or more discourses, either those of the narrator and character(s), or of different characters (q.v. "quasi-direct discourse"). "Heteroglossia" should not be confused with "polyphony." The latter term is used by Bakhtin primarily to describe Dostoevsky's multi-voiced" novels, whereby author's and heroes' discourses interact on equal terms. "Heteroglossia," on the other hand, foregrounds the clash of antagonistic social foces (pp. 248-49).