Definition
Composer is the dependency manager for PHP. In a Magento context, Composer installs Magento Open Source or Adobe Commerce, manages third-party modules from public and private registries, handles version updates, and resolves dependency conflicts. Composer install is the preferred delivery method for modern Magento modules.
What Composer does in a Magento store
The composer.json file at the store root lists every package required to build a working Magento. The composer.lock file pins exact versions so the same packages install identically across environments. Running composer install fetches everything into vendor/ for Magento to autoload.
Why Composer matters for module delivery
Composer-delivered modules version cleanly, support security patches via composer update vendor/package, and declare their dependencies explicitly. ZIP-delivered modules have to be manually re-installed at every upgrade. Most serious Magento extension vendors support Composer installation as the primary delivery channel.
Private Magento registries
Magento core packages live behind authentication on repo.magento.com. Each third-party vendor that sells via Composer typically runs their own private registry. The auth.json file at the store root holds the credentials. See the dedicated Composer authentication post for the working setup.