Most of the commands require root access (although you can send toast messages without it). Simply send a text to your phone in the format of "[email protected]" (no quotes) and the app will read it, then execute it. Here are some of the commands you can use:
Sending terminal commands: [email protected](pass) (terminal commands, seperate commands by new line)
Uninstalling apps: [email protected](pass) (package name of app)
echos: echo (what to put in toast, can be separated by new lines)
multiple echos multi-echo (what to put in the toast, new echos separated by new lines)
Bypassing android lockscreen security: [email protected](pass)
SDcard Wiping: [email protected](pass)
You can read more about the app at XDA here. For most people, this probably isn't going to be an everyday app, but if you fiddle about with recovery often, or you want that extra line of defence if your phone gets lost, SMSTasks can help.