What is asynchronous data transfer?

Synchronous= syn+ chrono = togetherness in time. So, the two or more communication system agree to be together in time to the change in signals or data.That means they should share the same clock speed.

In asynchronous systems, the clock information is contained in the data block itself in the form of start and stop bits and hence the receiving system can adjust itself to that speed,any speed. Async systems are serial in nature and the former can be either serial or are mostly parallel.