SMS stands for Short Message, or Short Messaging Service, and is simply the text messages your mobile phone may send and receive on a daily basis. CiviCRM can handle the sending and receiving of these messages from within the system, providing you with an extra means of making and tracking communication with your constituents, other than email.
SMS messages can be sent to mailing lists or the results of an advanced search for bulk SMS blasting, or a selection of individuals chosen manually.
If you wish to send a text message to an existing individual, open the contact's profile, click the "Actions" button and select the option "Send SMS". You can also create an SMS in the activities tab of a contact from the "- new activity -" drop-down menu.