SMS messages (also known as text messages) are a way to keep in touch with your customers or clients by sending them messages directly to their mobile phones. If they provide you their phone number, you can alert them to promotions, special offers, events, and the like even when they're not at their email. You can even send text messages to a User if you have a number in the Mobile Phone field on the user record.
You can send SMS messages through ClickDimensions both in bulk and to individuals and you can even receive messages in return (if you use Twilio).
There are 3 providers that you can choose from to do the actual sending of your messages - Twilio, MessageMedia (formerly MessageNet) and Bulk SMS.