With BDC it was not possible to update the data in the External Data. BCS allows to create, read, update and delete external data from SharePoint.
Two important concepts to understand with BCS are:
External List: An external list is a list based on data from an external system outside of SharePoint.This list can be created using SharePoint 2010 UI from Site Actions ->View All Site Content -> Create. Once this list is created we need to map this list with the External Content Type.
1. Workflows: We can not attach workflows with the items in the external list.
2. Item Permissions: There are no item level permissions in the external list.
3. Export to Excel: We can not export the content from the external list to excel.
4. Versioning: External List does not support multiple versions with the list items.
5. Item History: There is not item history.
6. Datasheet View: We can not see the list in the data sheet view.
7. RSS Feed: No RSS feed is supported on External List.
External Content Type: External Content Type is a new content type and it describes the schema and data access capabilities of an external data source. It also define what actions can be taken on the external data inside SharePoint. One of the coolest part of this is that External Content Type can be created using SharePoint Designer 2010 or Visual Studio.