What is the Full Form of MMS ?

MMS - Multimedia Messaging Service

Multimedia Messaging Service is a standard way to send messages that include multimedia content like images, audio file, video clips, rich text etc. The service can only be used in devices which support this service. These devices can include :
Mobile phone
Smart phones
Personal Digital Cellular
Handheld devices
or even Personal computers.

There are 2 kinds of services available in MMS. One requires GPRS which is quite slow, other uses 3G network, that can enable streaming video.

MMS - Microsoft Media Server

Microsoft Media Server is a Microsoft proprietary network-streaming protocol, serves to transfer unicast data in Windows Media Services.