From Oracle FAQ
DB2 or DB/2 is a DBMS system provided by IBM Corporation that competes with the Oracle database. DB2 is predominantly used on mainframe computers (i.e. OS/390). However, versions exist for AS/400, Unix, Linux and Windows as well.
Although DB2 seems to be quite a competent database platform, it doesn't have that big a market share outside its Mainframe stronghold.
Migration to/from Oracle
Oracle's Migration Workbench tool can be used to convert DB2 databases to Oracle.
- IMS - Information Management System