A multimedia story is some combination of text, still photographs, video clips, audio, graphics and interactivity presented on a Web site in a nonlinear format in which the information in each medium is complementary, not redundant.
Nonlinear means that rather than reading a rigidly structured single narrative, the user chooses how to navigate through the elements of a story. Not redundant means that rather than having a text version of a story accompanied by a video clip that essentially tells the same story, different parts of a story are told using different media. The key is using the media form - video, audio, photos, text, animation - that will present a segment of a story in the most compelling and informative way.
When news organizations take full advantage of two other important characteristics of storytelling on the Web -- context and continuity -- multimedia stories are wrapped in a story "shell" that provides background information on the story. This could include everything from databases, timelines and infoboxes to lists of related stories, links to other resources and online forums. The information in each shell gives the reader a sense of the context of a story and where it fits in with other stories on the same topic.
Each story shell, in turn, is part of a broader issue or beat shell at a news site that defines the more general context for each story - politics, foreign policy, education, crime, etc. And those issue/beat shells are wrapped in a general shell - usually a home page with its navigational menus - that defines what a journalism organization stakes out as its territory.