~Best wishes for the new year~
For those who are new to the BDC check out the channel 9 introduction video.
The BDC is all about the application definition file (XML File) that describe the metadata model for the backend system that you are connecting to. You can either create it manually or semi-automatically with tools.
I recommend to start learning the basic elements of an application definition before using tools. The reason for that is that you sometimes need to modify a definition after you have generated one. Especially the free MS BDC Tool (available in the MOSS 2007 SDK) has his shortcomings... but its a great tool to support your application definition development.
One of the best resources about the BDC on the net to create definition files manually: Blah!Blah!Blog!!
And some recent posts about the Business Data Catalog Editor:
-Using business data catalog Editor
-Working with business data column