Definition
A Magento extension is a packaged software module that adds, modifies, or replaces functionality in a Magento Open Source or Adobe Commerce store. Extensions are distributed via Composer or as ZIP files and installed into the Magento `app/code` or `vendor/` directory tree.
Extension vs module — same thing
In Magento terminology, "extension" and "module" are used interchangeably. Both refer to the same packaging concept: a unit of Magento code that can be added to or removed from a store independently.
How extensions are distributed
Modern extensions ship via Composer through a vendor's private or public package registry. Older or budget extensions still ship as ZIP files for manual installation. Composer is preferred because it handles dependency resolution and version management cleanly.
What extensions can do
Extensions can add new admin features, new frontend templates, integrate third-party services, change checkout flow, add payment methods, modify product display logic, and anything else exposed through Magento's plugin system or event observer pattern. The breadth of customization is one reason Magento ecosystems have a deep extension market.
Where to buy extensions
Three main channels: the Adobe Commerce Marketplace (official), independent module marketplaces (eTechFlow Modules, Amasty, Mageplaza, others), and open-source GitHub repositories. Quality varies more than price, so use the seven-question buyer's framework before purchasing.