An SMS Gateway is a way of sending a text message without using a mobile phone, and operates by forwarding a simple e-mail to a mobile phone recipient. When a Remote Notification recipient is configured as the SMS recipient type, a shortened version of the usual health alert email is sent in a mobile phone friendly format.
Emails sent to an SMS Gateway are typically in the format of phonenumber@smsgateway. For example, on the Sprint network this would be email@example.com.
Remote Notification is pre-configured with several of the more common SMS Gateways to facilitate the quick and easy setup of new recipients. If your mobile provider is not listed, please see this Wikipedia article for a list of many carriers and their settings.
See this Wikipedia article for more information about how an SMS Gateway works.