The bits are sent as quickly or as slowly as the sending side chooses. While bit-wise asynchronous communication is rare, some system transmit the bits within a byte in a synchronous manner while the bytes are asynchronous. Once a byte starts transmitting, each bit is sent one after another according to a clock. There is no synchronization between bytes. Another byte can arrive at any time.
Keyboards typically generate asynchronous data.
It is also known as ITU V.24. When there is nothing to send, the line is kept at the -15 volt state. This helps to differentiate between an idle line and when the connection has been broken which would drop the voltage to zero. The sender and receiver must synchronize on when a bit is being sent. To know when a new character starts, a 0 start bit is sent.