I learned about Summary Blocks from the Squarespace blog post about magazine-style layouts. I use a Summary Block to display featured posts in the sidebar. I also use them to create "Pinterest-style" galleries for a few category archives. They're really versatile. You can also use them to create recent post thumbnails or display your content as lists or grids. Check out the step-by-step tutorial and screencast at Squarespace. Below are the settings I use for the sidebar featured-posts and Pinterest-style galleries, if you're curious.