It all started in 1970 when a brilliant man named E. F. “Ted” Codd wrote a paper while working at the IBM research lab in San Jose, California. The paper, which was titled “A Relational Model of Data for Large Shared Data Banks” and this triggered a revolution on how the people thought about data in IT, its storage, and its management. And later they were called Codd's Rules. Other members of the IBM research team—Ray Boyce and Don Chamberlin—added to Codd’s work in the following years, developing “SEQUEL,” a language for structured English queries. This evolved into the “SQL” that is now the "language of RDBMS" used with most databases today.
As far as the bookish definition goes - DB2 is short for Database 2, a family of relational database products offered by IBM. DB2 provides an open database environment that runs on a wide variety of computing platforms. A DB2 database can grow from a small single-user application to a large multi0user system. As talked about earlier - using SQL, users can obtain data simultaneously from DB2 and other databases. DB2 includes a range of application development and management tools.
In addition to its offerings for the mainframe OS/390 and VM operating systems and its mid-range AS/400 systems, IBM offers DB2 products for a cross-platform spectrum that includes UNIX-based Linux, HP-UX, Sun Solaris, and SCO UnixWare; and for its personal computer OS/2 operating system as well as for Microsoft's Windows 2000 and earlier systems. DB2 databases can be accessed from any application program by using Microsoft's Open Database Connectivity (ODBC) interface, the Java Database Connectivity (JDBC) interface.
RDBMS from IBM.