When visually designing a website I used to sit down, open up Adobe Photoshop, and just start throwing visual elements at the page until something stuck. Often, actually always, the end result was completely different than from what I started with. I would start designing without a clear idea of what I wanted the site to ‘feel’ like, or worse yet what the client wanted the site to feel like. If you know the mood you want to convey before starting the design you can cut down the time it takes to build the site and also end up with a more cohesive and complete product.
Mood boards are a great way to express visually how you or your client would like a website to feel. Do you want a site that feels crisp and clean? A site that feels dark and grungy? Playful and fun? A mood board can be collage of images found in print media, a collection of color swatches, a collection of screenshots from websites highlighting certain looks. They are a great starting point that will save you time and frustration.