Cartridges in Endeca are prebuilt and modular components that pull in content and data (Videos, reviews, Analytics,...) from the Endeca engine and external system. Cartridges do expose Endeca features (guided navigation, search, spotlights).
In the Endeca package (application and SDK) you will find at the moment 22 cartridges OOTB.
A technical team can create custom cartridges if needed.
In the Experience Manager you can use cartridges to configure pages.