IM, also known as Instant Messaging,is a type of communication method that allows a server to send a private message instantly to another server via the internet. Instant Messaging can be used not just in one-to-one private messages but also group chats. There is different type of services you can use to use IM such as, MSN Messenger, iMessage (Apple), Facebook Messenger, WhatsApp, Yahoo! Messenger and more. It lets the server see whether the other server is online, has seen the message and is typing a message to them. As shown below, when the recipient is typing a message to you, you will see a little speech bubble with three dots.
As shown above, I like to use iMessage, an Apple service, for instant messaging as it is practical for me since most of my family members and friends use it. However, people without an Apple product are unable to receive messages from iMessage and will end up receiving an SMS message instead. iMessage enables the server to send text, pictures, videos and emoji to the recipient which makes it a great IM service to use.