It has a long history and was what some consider to be the first database product to use SQL. According to Michael Stonebraker, when IBM chose to make SQL the standard for database query languages by announcing its inclusion into DB2, Oracle seized the opportunity to trumpet that it too used SQL.
Currently, there is a dogfight going on between DB2 and Oracle for the number 1 position in the market. Historically, it is interesting to note that when Informix acquired Illustra and introduced make their database engine a object-relational database by introducing their Universal Server, both Oracle and IBM followed suit by changing their database engines to be capable or object-relational extensions. Technically, DB2 can be considered to be an object-relational database.
DB2 started out on the mainframe but soon migrated down through UNIX and Windows servers to PDAs.