Whilst text messaging only allows for plain, un-formatted text to be sent between compatible devices, multimedia messages can send and receive formatted text/pages, photos, sound clips in various formats and video; composition closely resembles an e-mail, with the use of "to", "cc" and "bcc" fields, plus support for delivery/read receipts.
The sort of content that can be sent and received depends on both your service provider, the sending and receiving devices; many service providers limit messages to 300-500KB, however the Multimedia Messaging Service (MMS) is capable of more than this, if the service provider allows it.
You need to have the relevant settings in your MMS-compatible device in order to receive multimedia messages via the MMS - usually your device will be pre-configured if you use it with the service provider it was designed for/you bought it from, however if this is not the case, your service provider's technical support department should be able to help you out (provided they have sold that model at some point)...
Posted from my CrackBerry at wapforums.crackberry.com