REDCap extensions are code customizations that exist independently of, but work in conjunction with, the official REDCap code base developed by Vanderbilt. This allows developers (Consortium members and our internal staff) to easily make use of REDCap's core features (e.g., authentication, MySQL database access, PHP variables, constants, and functions) to provide additional features and functionality that are not available in REDCap. The extensions come in a couple of flavors, officially known in REDCap as plug-ins, hooks and external modules, and can be used in conjunction with each other.
Details about available MGB REDCap Extensions can be found on the MGB REDCap Resource Center Extensions page.