CICS is Customer Information Control System.
CICS is used for Online processing.Online programs run under the direction of CICS. CICS is like an operating System within the operating system.
The following components and functionality define CICS :
DB/DC System
Teminal Oriented
Transaction Oriented
Pseudo-Conversation
Multi Tasking

Only one copy of the source code is loaded into memory and multiple users access it. This is called as Multithreading. For multithreading the program should be reentrant. Reentrant program is one which does not modify itself so that it can reenter itself and continue processing after an interruption by operating system.

CICS implements Quasi Rentrant technique. Seperate copy of Working storage is provided for each user.

One uint of work from CICS point of view is a task and from the terminal operator’s point of view is a transaction.