The Mathematica help system works with a highly structured set of documents. Each document falls into one of a number of categories, the most important of which are guide pages, reference pages, and tutorials. Each of these pages has a structure and style that matches that in Mathematica. These pages can intermingle with other pages that do not follow the Mathematica help style.
A guide page focuses on a single topic, providing links to functions that share functionality. Sections are provided to link to other related documents. A sample guide page is shown below.
A symbol or function page focuses on a single function or option, providing its different input templates with corresponding usage, additional notes including the function's options, if any, and a set of examples. Sections are provided to link to other related documents. A sample symbol page is shown below.
A tutorial page focuses on a particular piece of functionality and includes examples, tables with links and short descriptions of useful functions, and mathematical textbook-like information. Sections are provided to link to other related documents. A sample tutorial page is shown below.
An overview page provides a table of contents for other pages, particularly tutorials. A sample overview page is shown below; note that each entry in the overview page gives a link to a target page.
You can also add other pages to the Mathematica help system that do not follow the structured system. How to do this is documented in the alternative formats section.