The difficulty with asynchronous communications is that the receiver must have a way to distinguish between valid data and noise. In computer communications, this is usually accomplished through a special start bit and stop bit at the beginning and end of each piece of data. For this reason, asynchronous communication is sometimes called start-stop transmission.

Most communications between computers and devices are asynchronous.

Asynchronous communication
This page explains how asynchronous communication has adapted to new requirements, along with a brief history of this standard.