TYPO3 4.3|The upcoming stable release (for new projects)<br />This version is the next stable release and will be released in late 2009. When it will be released, this will become the recommended version for all new projects.
TYPO3 4.2|The current stable release (for existing projects)<br />This version is the current stable release. It is recommended for existing projects and have bug fixes until TYPO3 4.4 has been released.
TYPO3 4.1|The old stable release (for old projects)<br />This version is the old stable release. It will not be supported with bug fixes anymore after the release of TYPO3 4.3. Updating to TYPO3 4.2 is highly recommended.