Composer pages consist of several editable regions.
Page content describes the “meat” of the page. This would include all of the elements you add to a particular page and the content they hold. When you add an element to a page in Composer, you're placing that element on the "page content" region.
Banners appear at the top, bottom, left, and right sides of a page. Left and right banners flank page content and can contain their own elements. Top and bottom banners, called headers and footers, span horizontally across the top and bottom of a page, respectively. Each banner can include Composer elements. (Typically, headers and footers utilize dedicated elements specifically designed for a header or footer.)
There are 12 available page layout diagrams you can use for the page content, not including additional options provided by adding banners. Each layout provides a baseline grid with rows and columns that you can add Composer elements to. Learn about how to select banners and layouts for your page in Page Layout & Appearance.
Finally, the overall style and appearance of a page is determined by the page’s theme. Your site may come with a single default theme or multiple themes for different subsections of your site. Find out more about using themes on your site in Applying themes in Composer.