In a traditional sense, a style guide is much like a brand standard book for your website. It indicates the specifics of a brand, its colors, type, spacing of elements, and how to and how not to use the brand. It should include colors, type, forms, buttons, links, table, etc. and describe the interactions of those elements. Creating a simple look and feel is extremely important. These simple elements can be combined to create more complex groups.
We're used to working with complex and exciting brands. Colors are the eye-catchers of your site. Managing these colors all in one place is important to keeping your website system in sync. The last thing you want is for your editors to have free control over what colors are being used.
Typography is an art of arranging all the contents of your website, giving a feel to it and presenting the information in a professional way. Type is what helps tell the story of your website. With a style guide, your editorial team is equipt with the styles that will keep the site consistent.
Every site is complex and made up of numerous displays of content. The display of that content shouldn't be difficult for your editors to use. Components and sections are easy for editors to fill out and make viewable on the website.