Business Connectivity Services (BCS) allows you to hook up external data with SharePoint and Office applications, enabling either professional developers or citizen developers in your organization to quickly develop composite solutions. BCS is implemented as a SharePoint service application, which allows you to create an external system definition once, known as a Business Data Connectivity (BDC) model, and share these definitions with many sites. In addition, a SharePoint farm can host more than one BCS, and each one can be configured independently by different sets of administrators.
The BDC model consists of XML declarations that describe the external system you want to access as well as the external content type (ECT) that defines the operations you might like to use on this external data and its format. Once an ECT is defined, then using the browser or SharePoint Designer, you can manipulate the data from the external system as you do other SharePoint objects, such as lists and Web Parts, by using the external list, a new list type in SharePoint 2010.
You can extend the out-of-the-box BCS functionality using either client-side or server-side code. Client-side code can be developed using tools such as the browser, InfoPath Designer, or SharePoint Designer, whereas server-side code is developed using Visual Studio and deployed as precompiled dynamic-link libraries (DLLs).
SharePoint 2010 has tighter integration with Office client applications than in Microsoft SharePoint Server 2007 had with the Office client applications. You can use the BDC model to reveal external data in Microsoft Office 2010 applications, including Microsoft Outlook 2010, Microsoft Access 2010, Microsoft Workspace 2010, Microsoft Word 2010 and Microsoft InfoPath 2010.
Whether you create a SharePoint-related or Office client application BCS solution, you should consider the infrastructure and security implications, and plan the solution accordingly.