A lot of people are confused by the differences between Business Connectivity Services (BCS) and Business Data Connectivity (BDC), particularly by the acronyms and the mix of functionality. Todd Baginski is the best one to explain this, but here we go:

The Business Data Catalog (also known as BDC) existed in Microsoft Office SharePoint Server 2007 (MOSS, not Windows SharePoint Services).  It’s gone.

Business Data Connectivity (also acronym’ed BDC, though it’s now Connectivity, and it plays a very similar role) was introduced in SharePoint Foundation 2010 (also SharePoint Server 2010, of course).  This BDC’s job is to provide connectivity to back-end data systems, as was the job of the BDC (Catalog) in MOSS 2007.

However, it’s much more rich. Among its “big adds” are:

1. It’s in Foundation, so its capabilities are available “for free”

2. It supports read/WRITE access to data. To learn more, see these MSDN articles:

•          Business Data Connectivity (BDC) Service 

•          BDC Architecture   

Business Connectivity Services (BCS) is new in SharePoint 2010.  It provides access to data in SharePoint Foundation, SharePoint Server, and Office Clients. So all three actually have BCS, but it provides different functionality for each.

In Office clients (e.g., Outlook, Word, Access, InfoPath, SharePoint Workspace), there’s actually a layer of BCS running on your PC that allows you to connect to back-end data sources.  So you could, for example, expose your employee contact information stored in PeopleSoft as contacts in Outlook. BCS also allows taking stuff offline, which is awesome.

In SharePoint Foundation, BCS allows you to expose an external data source as if it were a SharePoint list—to interact with that data as if it were native to SharePoint (reducing training, etc.).  In SharePoint Server (Standard license), Search is able to index (and query) back-end data. With the enterprise license, you get Business Data Web Parts to expose back-end data in other ways, and you can connect the rich Office clients to SharePoint’s external lists. Take a look at these MSDN articles:

 •          Business Connectivity Services Overview 

•          What Is Included in Business Connectivity Services?

•          Understanding Business Connectivity Services

So it’s fairly safe to explain BDC as the “plumbing”—the core code.  BCS is a collection of functionality, exposed as services, that’s one thing on Foundation, enhanced by SharePoint Server (Standard and again by Enterprise), and yet another thing in Office clients.  BCS exposes and provides access to all the goodness that BDC is doing.