Tutorials are step-by-step cookbooks for common activities that incorporate several packages. They are more specific and less conceptual than Guides but more extended than Examples.
Guides are comprehensive, conceptually-focused documents providing stand-alone introductions to core packages in addition to the underlying astronomical concepts.
Documentation is the complete description of a package with all requisite details, including usage, dependencies, and examples.
Examples are stand-alone code snippets that live in the astropy documentation that demonstrate a specific functionality within a package.