WordPress Codex is an online manual for learning WordPress. It is a repository of information on how to use WordPress. It is a living document in the sense, it is updated frequently as per changes and new discoveries in WordPress. It is like a wiki where anyone can contribute.
It is divided into many sections for ease of use:
- Learn to use WordPress – Here you can learn how to use WordPress and all the functionality.
- Working with Themes – You can learn to create themes and more on design and layout
- Write a Plugin – This section contains information on plugins and how to create them. It has information on related APIs and resources
- Contribute to Development – The details related to how to contribute and support mechanisms are mentioned here. Information on Development, Documentation, Security and WordPress APIs is here.
- Give Back – This section has information on how to be part of the documentation team, details of documentation team and how one can volunteer or be part of the support team for Codex and WordPress. It has information on the various sections of Codex related to installation, themes, templates, plugins, troubleshooting etc.
You can click here to go through it.