1) In general, messaging (also called electronic messaging ) is the creation, storage, exchange, and management of text, images, voice, telex, fax , e-mail, paging, and Electronic Data Interchange ( EDI ) over a communications network.
2) In programming, messaging is the exchange of message s (specially-formatted data describing events, requests, and replies) to a messaging server , which acts as a message exchange program for client programs. There are two major messaging server models: the point-to-point model and the publish/subscribe model. Messaging allows programs to share common message-handling code, to isolate resources and interdependencies, and to easily handle an increase in message volume. Messaging also makes it easier for programs to communicate across different programming environments (languages, compilers, and operating systems) since the only thing that each environment needs to understand is the common messaging format and protocol.
IBM's MQSeries and Sun Microsystems Java Message Service ( JMS ) are examples of products that provide messaging interfaces and services.