The two are closely related, but are not the same thing. The focus of data management is mainly cost related and deals with reducing data volumes, regardless of the contents of that data. It involves four basic approaches for keeping data volumes in check:
To be able to implement an ILM strategy you need a good data management strategy as a foundation. SAP ILM tries to achieve a good balance between TCO, risk, and legal compliance. So in addition to managing data volumes, SAP ILM also manages data retention requirements, including such things as the final destruction of information.
SAP ILM is the sum of all of these measures. With regard to the origin of the data ILM covers both live systems and legacy systems. For legacy systems ILM provides methods and tools to extract data from a system to be Decommissioned, and move it to the Retention Warehouse, where it can be accessed in case of reporting and auditing requirements.