Screaming Penguin - Source Does Matter

"SMIL is the Synchronized Multimedia Integration Language, a language designed to create interactive multimedia presentations using various types of media, including audio, video, images and text. Based on XML, SMIL allows developers to synchronize and coordinate the presentation of different media files, thereby making it possible to easily and quickly create Web-based multimedia offerings."

The linked artile on devShed has more, basically SMIL is a standardized and non proprietary presentation format (as opposed to Flash, etc).
  Learning To SMILe: devShed