The data transfer workbench is a central transaction for the automatic transfer of data from a legacy system to an SAP system. It provides the tools required for the initial and any subsequent data transfer.
The complete documentation on the data transfer workbench is in the SAP Library:
CA - Cross-Application Components -> CA - Data Transfer Workbench
The data transfer is described step by step and there is also a description of object specific features that you must take into account.