Halbwachs introduced the now common term of "collective memory" to indicate a type of memory which is shared by an entire society. Through collective memory one member of society can recall and re-experience an important event that he did not himself take part in. collective memory according to Halbwachs does only tell members of a society what they remember or can remember but also how to remember and conceive various events in the history of a social group.
What is interesting in the concept of collective memory by Halbwachs is that each and every member of society takes part in building and shaping collective memory. Thus, personal memory, what Halbwachs calls autobiographical memory, is never truly personal because it is always in some sort of relationship with the collective memory. Another type of memory contrasted by Halbwachs to collective memory is historical memory. Unlike collective memory, historical memory is "objective" and "scientific" (according to Halbwachs rather naïve take on it), which is written and ordered. Collective memory, on the contrast, is never formally articulated nor is it never a stable thing which is unified across all members of society. It is important to note that there is no one collective memory, and that the collective memory is in actual fact a collection of collective memories or various perspectives and standpoints towards collective memory.
An interesting situation regarding Halbwachs theory is when different types of memory contradict. For instance when the personal, autobiographical memory of an event such a war as something traumatic contradicts with the collective memory of that war as something heroic and important. Another possible clash is between the fluid, undecided and often highly emotional collective memory and the accuracy seeking historical memory. For example, historiography can relate to various events in a very different fashion compared with popular sentiment regarding it.