There are many forms of communication. For most of them we can distinguish between asynchronous and synchronous communication.
Synchronous Communication describe all forms of communication, where the sender and receiver communicate instantly. Messages transferred between sender and receiver therefore don't have to be saved or archived somewhere else, but get exchanged directly.
Forms of synchronous communication are telephone and video calls and face-to-face conversations. Instant messaging is often put in-between asynchronous and synchronous communication, as it can be used both ways.