A Keyword (corehq.apps.sms.models.Keyword) defines an action or set of actions to be taken when an inbound SMS is received whose first word matches the keyword configuration.
Any number of actions can be taken, which include:
Replying with an SMS or SMS Survey
Sending an SMS or SMS Survey to another contact or group of contacts
Processing the SMS as a Structured SMS
Keywords tie into the Inbound SMS framework through the keyword handler (corehq.apps.sms.handlers.keyword.sms_keyword_handler, see settings.SMS_HANDLERS), and use the Reminders framework to carry out their action(s).
Behind the scenes, all actions besides processing Structured SMS create a reminder definition to be sent immediately. So any functionality provided by a reminder definition can be added to be supported as a Keyword action.