BCS: Business Connnectivity Services helps bringing data into sharepoint and office. It also works online/offline and supports both client/server and read/write
BCS Sources contains: DB Connector, WCF/WS connector, Custom Connector and .NET assembly connectors.
Three Steps from BDC to BCS
1) Discover: Discover is finding your source: DB, WCF/WS, .NET assemblies
2) Map: is defining key actions (CRUD)
3) Connect: Connect is display through External Content Types, Contacts, etc.