Business Data Connectivity (BDC) service is the new version of the Business Data Catalog that was provided in Microsoft Office SharePoint Server 2007. It is now provided in Microsoft SharePoint Foundation 2010. BDC is implemented as a shared service in Microsoft SharePoint Foundation 2010 and it still relies on the Metadata Store. The BDC Service enables you to declaratively model external systems so that you can expose external data in SharePoint Server 2010 (and, with Microsoft SharePoint Server 2010 with Enterprise Client Access License, in Microsoft Office 2010 applications also). BDC bridges the gap between SharePoint sites and your business applications, and enables you to bring in key data from various external systems to SharePoint lists (via the new external lists feature and external data columns), Web Parts, search, user profiles, and custom applications.
Questions tagged bdc regards the implementation, use and maintenance of Business Data Connectivity.
Reference: Business Data Connectivity (BDC) Service