Stands for "Short Message Service." SMS is used to send text messages to mobile phones. The messages can typically be up to 160 characters in length, though some services use 5-bit mode, which supports 224 characters. SMS was originally created for phones that use GSM (Global System for Mobile) communication, but now all the major cell phone systems support it.
While SMS is most commonly used for text messaging between friends or co-workers, it has several other uses as well. For example, subscription SMS services can transmit weather, news, sports updates, and stock quotes to users' phones. SMS can also notify employees of sales inquiries, service stops, and other information pertinent to their business. Doctors can receive SMS messages regarding patient emergencies.
Fortunately, text messages sent via SMS do not require the recipient's phone to be on in order for the message to be successfully transmitted. The SMS service will hold the message until the recipient turns on his or her phone, at which point the message will be be sent to the recipient's phone. Most cell phone companies allow you to send a certain number of text messages every month for no charge. Though it would be a good idea to find out what that number is before you go text message crazy.