1. A method for determining a semantic concept classification for a digital video clip including a temporal sequence of video frames and a corresponding audio soundtrack, comprising: a) receiving an audio-visual dictionary including a plurality of audio-visual grouplets, the audio-visual grouplets including visual background codewords representing visual background content, visual foreground codewords representing visual foreground content, audio background codewords representing audio background content and audio foreground codewords representing audio foreground content, wherein the visual background codewords, the visual foreground codewords, the audio background codewords and audio foreground codewords in a particular audio-visual grouplet were determined to be correlated with each other during a training process based on an analysis of a plurality of training video clips; b) using a data processor to analyze the temporal sequence of video frames to determine a set of visual features; c) using a data processor to analyze the audio soundtrack to determine a set of audio features; d) determining similarity scores between the digital video clip and each of the audio-visual grouplets by comparing the set of visual features to any visual background codewords and any visual foreground codewords associated with a particular audio-visual grouplet, and comparing the set of audio features to any audio background codewords and any audio foreground codewords associated with the particular audio-visual grouplet; e) determining one or more semantic concept classifications using trained semantic classifiers responsive to the determined similarity scores; and f) storing indications of the one or more semantic concept classifications in a processor-accessible memory.