Summary: 

Difference Between Instant Messaging and Text Messaging is that text messaging, also called SMS, is a capability that allow users to send and receive short text messages, typically fewer than 300 characters, on a phone or other mobile device or computer. 

Instant messaging (IM) is a real-time Internet communications service that notifies you when one or more people are online and then allows you to exchange messages or files or join a private chat room with them.

Instant messaging (IM) is a real-time Internet communications service that notifies you when one or more people are online and then allows you to exchange messages or files or join a private chat room with them. 

Real time means that you and the people with whom you are conversing are online at the same time. Some IM services support voice and video conversations. For IM to work, both parties must be online at the same time. Also, the receiver of a message must be willing to accept messages.