Similar to the Bank of America text banking, you type and send the word "BAL" to the BoA Text banking number and it responds with your balance.
How is this setup? Thank you.
If you don't mind using a long code (10 digit normal phone number), you can do it with a GSM modem hooked directly to your server. But that has some redundancy and reliability issues. There is free software to support this though: http://www.kannel.org/ http://www.gnokii.org/
If you don't mind your users needing to use a keyword at the beginning of all your messages, like "BofA BAL", you can use a shared shortcode. That's a lot cheaper and there are a bunch of providers who can supply that service.
Sorry there's not too much specific here. It's a big topic. But if you google around using some of these terms, you should find more info. Or respond back with some more specific questions.