Synchronous indicates that the communicating devices are using a common, active clock.
Asynchronous indicates that there is no common clock, and so some method must be used to let the receiver know that information is arriving, and where the valid bits of information are in the transmission.
In Ethernet, this is done with a preamble, which is initially a string of 64 alternating zeros and ones. This sequence announces that traffic is arriving, and give the receiver "the rhythm" to sync up.
In RS232, it's a simple "Start Bit;" it wakes up the receiver, and tells it to begin paying attention to the line transitions indicating marks & spaces.
In a synchronous communication, the clock is always present, payload areas are defined, as well as administrative areas (like framing and possibly in-band messages). Every clock cycle has a meaning and a purpose.
Because of the need to put in synchronizing elements, asynchronous communications have more overhead.
For example, two 64K lines, one sync, one async, the async line will have more transmission overhead (less payload for the time measured) because of the synchronizing elements.
Hope this helps,
Good Luck
Scott
