TYPO3 has extensive documentation. This documentation is linked here, so people can find stuff that is not documented here.

Main Documentation

The documentation of TYPO3 CMS is collected at docs.typo3.org, there is a list of all documentation for the core, and documentation for extensions.

Prominent documentation areas are

• TypoScript reference (tsref). A detailed documentation of TypoScript.
• TCA reference. A detailed documentation of the TCA.

API documentation

The API documentation for different versions of TYPO3 CMS is very useful when contributing to the core or creating an extension.