A mood board is often used to help paint a picture / scenario / idea / topic using visuals and words. In the case of advertising and marketing, the mood board plays an important in explaining core ideas, audience profiles, and possibly research findings as well using images related to a particular topic and put them together in a collate form.
i must say, the Ad 340 batch for Nov 2010 – Feb 2011 has done a really good job in their Mood Board preparation. here’s some photos and videos of the mood board as reference.