In the traditional UNIX model, I/O appears to be synchronous, as if you were placing a remote procedure call to the I/O device. Once the call returns, then the I/O has completed (or at least it appears to have completed--a write request, for example, might merely result in the transfer of the data to a buffer in the operating environment).
The advantage of this model is that it is easy to understand because, as a programmer you are very familiar with the concept of procedure calls.
An alternative approach not found in traditional UNIX systems is the asynchronous model, in which an I/O request merely starts an operation. The program must somehow discover when the operation completes.
This approach is not as simple as the synchronous model, but it has the advantage of allowing concurrent I/O and processing in traditional, single-threaded UNIX processes.