A short code is a 5 or 6 digit number that can receive incoming text messages. They are also called common short codes. What ever shortcode you register will work across all wireless carriers.
Registration in the U.S. is handled by the Common Short Code Administration. You can either get a random code which cost $500 per month, or pick your own vanity code for $1,000 per month.
Once you obtain your CSC, you must work directly with the carriers, or through an aggregator. An aggregator has links to the carriers so you only have to deal with one company
In most instances it will be cheaper and easier to get what is called a shared short code. There are companies that will give you keywords on their codes, and will handle the routing of messages to y our servers. This is a much more affordable way to sms enable applications. For more information or to get a shared short code, click here.
If you are interested in doing premium sms, where you can charge per message on the user's mobile phone bill, you can do that with either solution. You will need access to an sms gateway to send the text messages, which you can do with any bulk sms provider, or aggregator.