SMS is text marketing where folks just text a keyword to a short number... sort of how they did in the haiti disaster, ie: text Haiti to 55423 to donate 10 bucks.
Instead you'll have a sign, advertising space, etc. that says something like:
To Get Your Grass Cut
When people text the keyword "Mowing" to the particular number, their phone number is stored in your database where you can text or call them directly.
It's basically traditional list building but with phone numbers instead of emails. There's also an option to send out promos via mass texts to everyone who "opts-in" to your text list.
If that makes sense.
I'm considering implementing this, but haven't decided whether I will or not.