Structures are a very powerful feature that allow you to create headers, footers, and other site-wide content for your site. They can also be used to create archive and 404 pages.
Creating a Structure
You can create a new Structure by clicking the "New" button in the toolbar, and then clicking "New Structure," or by using the keyboard shortcuts, "n", then "c".
Designating the Content Area
Unless you are creating an archive page (for categories or tags, for example), or a 404 page, you will likely want to designate an area in your Structure for the page-specific content to go. This can be done by adding the "Content" module to your structure. You may also want to make the Row containing the Content module full width and remove its padding.
Setting Default Structures
You can set default structures for a particular content type. For example, if you want your blog posts to have a different Structure than the rest of your site, you can give them their own default structure.
Assigning Custom Structures
You can also assign a particular page or post a specific Structure if you don't want to use the default. In the toolbar, click "Edit," then click "Apply New Structure..." or use keyboard shortcuts, "e", then "n".