Difference between core and extensions

The TYPO3 base system is called the core. The functionality of the core can be expanded, using extensions. A small, selected number of extensions (the system extensions) are being distributed as part of the TYPO3 core. The core and its system extensions are being developed by a relatively small team (40-50 people), consisting of experienced and skilled developers. All code being submitted to the core is reviewed for quality by other core team members.

Currently there are more than 5500 extensions available in the TYPO3 Extension Repository (TER), written by some 2000 individual programmers. Since everybody can submit extensions to the TER, the code quality varies greatly. Some extensions show a very high level of code quality, while others have been written by amateurs. Most of the known security issues in TYPO3 have been found in these extensions, which are not part of the core system.