In what way synchronous and asynchronous serial modes of data transfer differ?

In the synchronous serial mode, the sender and receiver share a common clock, and data bits are sent continuously in blocks (frames) without start and stop bits for each character. It is fast, but needs clock or synchronising characters.

In the asynchronous serial mode, there is no common clock. Each character is framed by a start bit and one or two stop bits, so the receiver can synchronise on every character. It is simple and cheap but has extra bits as overhead, so it is slower. A keyboard link is an example.