You can choose to communicate with your customers using either of long codes, short codes, sender IDs, and keywords. Here is a brief description of the differences between each of these:
Long codes, generally referred to as standard phone numbers, are 10 digit numbers used to send and receive SMS messages and phone calls. You can easily buy new new phone numbers using IMIconnect as explained here.
Short codes are 5, 6 or 7 digit numbers that are used to send and receive SMS and MMS messages. Short codes are often the preferred choice for receiving high volumes of messages. For more information on short codes refer here.
Sender ID is a name or number that is used to identify the sender of a message. These can be numeric or alphanumeric. Not all countries support alphanumeric sender ids. Where supported, you can use it to aid brand recognition or build trust by using your company or brand name as the sender ID. Refer here for more information.
SMS keywords are specific words that can be used at the start of the SMS body text to identify the purpose of the incoming messages. They allow you to use the same short code for launching multiple services by associating a different keyword with each service. E.g. You can ask your customers to text-in BALANCE to fetch their latest account balance, and LOAN to request information about a loan offer while sharing the same short code. For more information on SMS keywords refer here.