Multimedia Messaging Service
An enhanced transmission service that enables graphics, video clips and sound files to be transmitted via cellphones. Developed as part of the 3GPP project, MMS phones are generally backward compatible with SMS and EMS.

Methods of delivery
There are two modes of delivery in MMS: immediate or deferred:

Immediate delivery: When the MMS client on the mobile phone receives the MMS notification, it then immediately (without user intervention or knowledge) retrieves the MMS message from the Multimedia Messaging Service Center (MMSC) that sent the notification. After retrieval, the subscriber is alerted to the presence of a newly arrived MMS message.

Deferred delivery: The MMS client alerts the subscriber that an MMS message is available, and allows the subscriber to choose if and when to retrieve the MMS message.
As with the MMS submission, the MMS retrieval request, whether immediate or deferred, occurs with an HTTP request. The MMSC responds by transmitting the MMS message in an HTTP response to the MMS client, after which the subscriber is finally alerted that the MMS message is available.

The essential difference between immediate and deferred delivery is that the former hides the network latencies from the subscriber, while the latter does not. Immediate or deferred delivery are handset dependent modes, which means that the handset manufacturer can provide the handset in one mode or the other or let the user decide his/her preference