The data transfer workbench is a central transaction for the automatic transfer of data from a legacy system into the R/3 system. It offers the tools required for the initial and subsequent data transfer.
The complete documentation about the data transfer workbench is in the SAP Library:
CA - Cross-Application Components -> CA - Data Transfer Workbench
The documentation tells you step by step how to carry out the data transfer, and which object-specific special features you should be aware of.