There are many forms of communication. For most of them we can distinguish between asynchronous and synchronous communication.
Asynchronous Communication (often referred to as "Async") is a form of communication that doesn't happen instantly. Contrary to synchronous communication – for example phone calls or a face-to-face talk – async communication is usually archived or saved somewhere for future reading. The participants therefore don't need to be present to receive the message.
Popular examples of asynchronous communication are e-mails, handbooks/wikis, letters or voicemails.