SMPP is a protocol for sending and receiving large numbers of SMSs.
You can rent space on an SMS Gateway server attached to your mobile operator's network, which will have an SMPP interface. You get a mobile number, to which your subscribers can send their messages.
Then you need to have your own server program to receive the SMSs in SMPP format; query your database; build a reply SMS and send it back to the gateway in SMPP format.
The SMPP is explained on Wikipedia here. At the bottom of the page are links to the latest versions of the SMPP spec.