I am sure all the 'cool' people know this already.
Situation: you work in an office. Your spouse works on the road. Since they are always driving around or on the phone it is hard to reach them. You just want to let them know that dinner is at 5:00 instead of 6:00 or that they need to get milk at the store.
Technology to the rescue. You can go onto your computer and email them. Simply type in a very brief message and send it to their phone number @ their phone company. If their phone company is Verizon, the address would be 1234567@vtext.com.
This could be a great time saver if you are playing 'phone tag' and just have a brief message. They can also reply to the text message pretty easily.
How precisely to do this depends on what phone provider they have, the type of phone, etc.