BCS or Business Connectivity services is a new and much improved, and much much enhanced version of what used to be BDC in SharePoint 2007.
It allows external data to be surfaced inside of SharePoint. But compared to BDC, it comes with much richer
a) Presentation options – in both server and client.
b) Tooling support – In SharePoint designer and Visual Studio
c) Connectivity options – in conjunction with the secure store service.
In my newest article on BCS, I talk about BCS in SharePoint 2010 with Visual Studio 2010. Hope you like it.
There is also a whole chapter (Chapter #9) devoted to BCS in my book.