SMS (Short Message System) you can sent only text by this method, pictures and videos cannot be sent by this system, if your carrier charges you by messages, it's great to know how big the message is, more expansive it will become, a simple SMS have only 160 characters (letters and spaces), if you pass this number, you'll be charged for another message and this way continuing the value
MMS settings are for Multimedia Message System.
You can send Pictures and in some cases Video and audio (some audio are not available on iPhones).
It's a total diferent way to receive and send MMS from SMS, because has a different setting, it use a Multimedia account on your settings...
All iPhones have the MMS and SMS settings automatically on the process you put the SIM card.