Google Instant Preview is combined with Instant Search to give you a full page thumbnail of the website you are about to visit. The thumbnail previews show up beside the search results, and can be toggled on or off, depending on your need for the feature.
Google Instant Preview may not be available for everyone, but you can opt-in through this page here and begin using the feature right away. Instant Preview shows both cached and up-to-date page previews from services like Twitter, where it displays messages on top of the preview, depending on the search result.
If you"re using Instant Preview, you can scroll through the search results with your mouse or arrow keys, hitting up or down to preview through the search results.
Google has been making some major headway with their search engine over the last month, even introducing Instant Search for mobile. Now mobile users can get the same desktop experience of Instant Search on their phone.