A mood board is extremely useful for establishing the aesthetic feel of a web site. It usually fits into the process somewhere after wireframes and before design mockups. Things that can be explored in the mood board include photography style, color palettes, typography, patterns, and the overall look and feel of the site. Soft or hard? Grungy or clean? Dark or light? A rough collage of colors, textures and pictures is all it takes to evoke a specific style or feeling.
The mood board is intentionally casual; it lets the designer start with broad strokes and get feedback before too much time is invested in the wrong direction. Think of it as rapid visual prototyping.