What is the difference between synchronous and asynchronous?

Sychronous transmission does not add start & stop bits at the beginning and end each byte transmitted for sending data segments while Asychronous transmission uses start & stop bits which are added at the start & end of data blocks.

Sychronous transmission provides 64 or 128 Kbps speed but

asychronous transmission provides 38.4 Kbps speed.