Business Connectivity Service (BCS) is next generation of the BDC (MOSS-2007). BCS is one of the Service Application in SharePoint 2010. please see the below screenshot. you can get here from CA->Application Management->Manage service applications.
BCS is included in Microsoft SharePoint Server/Foundation 2010 and the Microsoft Office 2010 . Using BCS we can connect to external Data sources such as SAP, Siebel, Oracle,Web Services (.ASMX, WCF), .NET class. It's all about bringing external data from back-end business systems into SharePoint so that end users can work with the data in a SharePoint environment. The difference between BCS and BDC (MOSS 2007) is using BCS we can do CURD operations on External System. but with MOSS 2007 BDC we were not able to do this operations.
External Content Types:
A core concept of Microsoft Business Connectivity Services (BCS) is the external content type. External content types are reusable metadata descriptions of connectivity information and data definitions plus the behaviors you want to apply to a certain category of external data. External content types enable you to manage and reuse the metadata and behaviors of a business entity such as Customer or Order from a central location, and enable users to interact with that external data and processes in a more meaningful way. for more info please refer the following http://msdn.microsoft.com/en-us/library/ee556391(office.14).aspx
Microsoft BCS has built-in support for connecting to the following types of data sources:
WCF and Web services
Microsoft .NET Framework assemblies
Custom data sources;
Next Blog post I am going to show how to connect to External DS and bring data to SHAREPOINT using SHAREPOINT DESIGNER , .NET Connector and WCF ,.ASMX web Service.