TYPO3 is a very community-built system. There are a great number of resources available to help us, and also, to expand and enhance TYPO3.
The typo3.org website is the main resource for TYPO3 users. On this site we have:
Mailing Lists
Mailing List Archives
The facility to ask developers directly
IRC chat
Documentation
Bug reports
Videos
The documentation section of the typo3.org website, http://typo3.org/documentation/, contains a huge amount of documentation; there are guides to installing, getting started, templates, and TypoScript.
There is also a large number of videos available, mostly created by Kasper, showing how to use many of the features of TYPO3: http://typo3.org/documentation/videos/wmv-format/. These videos are a little old, and in some cases may be using slightly older versions of the software, but they are still useful in most cases.
This is an incredibly useful resource on the website. The mailing list archives, http://lists.netfielders.de/pipermail/typo3-english, are full of requests, questions and support that have already been responded to. If the archives don't have what you're looking for, then there are the active lists. There is a list for almost every possible aspect of TYPO3, from a community snowboarding tour to extension development, or just the English users' mailing list. The lists are available at http://lists.netfielders.de/pipermail/typo3-english.
TYPO3 is very extensible. Huge features and capabilities can be added at the click of a button. These extensions are created by the TYPO3 community members and are mostly accessible through the internal extension manager of TYPO3 itself. All extensions are available on the extension page, http://typo3.org/extensions/, and documentation for individual extensions is available with the extension itself.
Extensions have a varying degree of stability. Some extensions are completely stable and safe to use while others have not been completely tested and may contain bugs, such as alpha and beta versions of extensions.
There is also documentation on the website for creating extensions, along with coding guidelines and naming conventions.
Although TYPO3 is free, it relies heavily on sponsorship and donations. The involvement of Kasper and some of the team depends on if they can afford to focus time on the project. A number of large features, such as the Database Abstraction Layer, are commercially sponsored. Companies who wish to use TYPO3 and a particular undeveloped feature fund its development. Since they will be using it in a commercial environment, it seems fair to give something back.
In a similar respect, the TYPO3 community believes that those who use TYPO3 and earn money from such use (such as web developers using TYPO3 to power a client's website) should donate a percentage of their profits to TYPO3.