IMS is a Transaction
Manager and Database Manager from IBM
It was developed by IBM in the 1960's as the database management system for Rockwell and the Apollo space program.
IMS is the world's fastest transaction manager and has been tested at 11,000+ transactions per second on a S390 mainframe.
It is often compared and contrasted to CICS.
The DL/I (Data Language/I) database is a hierarchical database manager unlike DB2 a relational database management system RDBMS. Modern IMS TM systems will tend to use DB2 as their database.
Despite being an old dinosaur, IMS has learned a few new tricks in recent years - it is now possible to write IMS programs in Java.