SAP messages are very simple to create and maintain using transactions SE91 or SE80. First create a message class then add as many messages as you require into it. For each message you also get the option to create long text for it which offers the user additional information about the particular issue. Once you have created your message you can use them within your ABAP code using the following syntax:
MESSAGE = ABAP statement
E = Type of message (can also be I,W,S,A,X)
001 = Message number
MCLASS = Message Class
Further information about message types
I - Information messages: Information message are displayed in a popup window and only pause processing until you have clicked through the message.
E - Error message: An error message will be displayed in red along the footer of the SAP screen and stops processing going any further.
W - Warning message: A warning message behaves similar to and error message and is displayed along the footer of the SAP screen and stops processing going any further.
S - Success/Status message: A Success message is also displayed along the footer of the SAP screen but does not stop processing going any further and is simply displayed at the end in green.
A - Termination/Abend: A termination message stops processing and causes a runtime error and short dump which is also viewable in tcode ST22.
X – Exit: An exit message stops processing and displays and exit button which exits session processing. Similar to typing /n into the SAP command box.