On July 20, 1969, the human race accomplished its single greatest technological achievement of all time when a human first set foot on another celestial body. Johannes Kepler and IMS helped to bring people to the moon.
IMS stands for - Information Management System. It has two systems - a Database Manager (IMS DB) and a Transaction Manager (IMS TM). Usually either IMS DB or IMS TM might be referred to simply as IMS but you might not get the meaning unless you are working with zOS. Following the introduction of DB2, it was assumed, both inside and outside IBM, that IMS DB would die a slow death as DB2’s performance improved to approach that of IMS DB. But, it never happened, mainly because of the number of major IMS-based applications at large organizations worldwide.
IMS Database component stores data using a hierarchical model, which is quite different from IBM's later released relational database, DB2. In IMS, the hierarchical model is implemented using blocks of data known as segments. Each segment can contain several pieces of data, which are called fields. For example, a customer database may have a root segment (or the segment at the top of the hierarchy) with fields such as phone, name, and age. Child segments may be added underneath another segment, for instance, one order segment under each customer segment representing each order a customer has placed with a company. Likewise, each order segment may have many children segments for each item on the order.
Unlike other databases, you do not need to define all of the data in a segment to IMS. A segment may be defined with a size of 40 bytes but only define one field that is six bytes long as a key field that you can use to find the segment when performing queries. IMS will retrieve and save all 40 bytes as directed by a program but may not understand (or care) what the other bytes represent. In practice, often all data in a segment may map to a COBOL copybook. Besides DL/I query usage, a field may be defined in IMS so that the data can be hidden from certain applications for security reasons. The database component of IMS can be purchased standalone, without the transaction manager component, and used by systems such as CICS.
IBM's Hierarchical DBMS and Transaction Manager (IMS DC).