Messages are single texts, stored in table T100 , that you can maintain in Transaction SE91 or by forward navigation in the ABAP Workbench. T100 has the following structure:
Twenty-character message class
Message text (up to 72 characters)
Work areas assign messages to an application (maybe to a package, for example), and the message numbers identify the individual messages within the work area. When you call a message in ABAP, you need to specify the work area and message number. The language key is supplied by the system according to the system language environment.
For information about creating message classes and messages, refer to the Defining Message Classes section of the ABAP Workbench documentation