A Short Code (also known as Common Short Code or CSC) is significantly shorter than a full telephone number and can be used to Premium Bill SMS messages. They are designed to be shorter to read out and easier to remember than a normal length telephone number. 

While each carrier/operator maintains their own numbers, they generally have agreements in place to avoid overlap and to allow content providers to use the same number on different networks. Short code services can not be accessed by mobiles that are off-net (eg in a different country).