F.A.Q. pages are the most standard way online for informing and guiding users to help them find the answers to their questions about the service/app.
phpMyFAQ is a pretty stable and open source PHP F.A.Q. application that has many features for building a good F.A.Q. system.
A powerful admin interface exists for managing the categories, entries, users and also viewing the stats.
It is possible to enable users to ask or answer questions and comment to them or keep it admins-only.
There is an advanced search system empowering users to locate the related answers with ease.
The F.A.Q. page has some ready-to-use modules like “most popular, latest or sticky questions”.
Its look and feel can be customized with HTML-CSS, interface has multi-language support and all questions-answers can be exported as a PDF (with automated table of contents).