All of them are wireless messaging standards. SMS is the GSM "Short Message Service". It's very similar to "paging" in the US. This means, mobile phone users can "chat" with each other via SMS. SMS is limited to pure text messages of 160 characters max.
EMS (Extended Message Service) is an evolution of SMS. It adds formatted text, and uses bold or underline for emphasis.
MMS (Multimedia Message Service) is more powerful than EMS even, it can transport pictures, video clips, and other attachments.
This was first published in November 2001