TYPO3 has been used by small business as well as giants like CISCO, M TV, Airbus for managing their web content.
TYPO3 extensions are components developed on top of TYPO3 core for your business specific needs.
TYPO3 by default provides all services needed to run a basic website with high quality standards and re-usability using API.
It allows to develop custom functionality specific to your needs which seamlessly integrates with TYPO3 framework.
Extensions can be developed by anybody having knowledge of TYPO3 core APIs and php development skills.
It provides "extension kickstarter" which is useful as easy to start tool for development.
Extension follows a structure with which it fits seamlessly in the TYPO3 framework and makes a homogeneous setup of the website.
Backend provides Extension Manager which helps to install and uninstall extensions.
Repository provides hundreds of extensions which are easy to download and install API provides service to custom extensions built by individual developer of community.
API are set classes written in core which can be extended further for custom extensions.
Further more extension classes can also be extended to make new extensions from existing one.
Repository consists of hundreds of free to use extensions.
Open source extensions is most popular part of using TYPO3.
At this point there are around 4000 open source extension available in Repository.
Who creates them? Open source extensions are created by developers and enthusiasts all around the world.
How can I trust them? TER extensions are routinely reviewed by core group in order to keep quality as well as report potential security risks.
These issues are reported by security bulletin.
Most popular extensions are often updated to fix any issues.
Moreover you can download the source code and review yourself for any of your security concerns.