An SMS (per se) can only be sent from a cellular device. It's one of the most reliable forms of communication but only became popular by accident. They're limited to 160 characters, which is the available 'space' of bandwidth between voice transmissions over a cellular network. Mobile providers realised that money could be made by selling this 'space' as messages - it was mainly young people who caught on to this cheap way of communicating which made it so popular. It is thought that providers initially made more money from the sms than call charges!
Hangouts, Viber & others provide 'sms' services, but these can be as long as you wish, and rely on a data connection or wifi to send and receive - but they are not an sms in the true sense of the word. Usually a telephone number is used only for ease of identification, but is not necessary and is not confined to telephone numbers.
I hope the sms never disappears as it is 99% reliable, needs very little power and only a tiny signal - invaluable in emergency situations!
Sent from my Nexus 7 using AC Forums mobile app