dpeterc describes the essential difference, but I believe he is not quite correct on one minor point. It is true that XFlush just sends requests to the X server. This function is useful in normal application programs only when needing to flush Xlib graphics requests before doing some other operation; usually, applications are in an event loop, and XNextEvent automatically flushes. And XSync both flushes the output buffer and waits until all requests have been received and processed by the X server. There isn`t an application-level acknowledgement that the server has handled the requests, other than the function`s returning; it returns without a value, and there is no RequestsHandled event. Practically speaking, there is no difference in your application with this extra information.

Note that XSync handles the requests and generates events -- it doesn`t handle the events, of course, which you need to do in your application as part of the normal processing. This is why Xt programs, in particular, which call XSync and expect to have the screen updated don`t work properly -- the requests have gone to the X server, but the Xt program itself has not dispatched the resulting events to the appropriate widgets.

XSync is often useful in debugging, especially when a program is generating protocol errors. You can use -synchronize for an Xt program or make an explicit call to XSynchronize; you can also set _Xdebug to 1 in a debugger. Then you can set a breakpoint in the error handler to check the traceback to the offending line of code in the application -- getting the error as it occurs.

Man pages are at http//www.motifzone.com/resources/man/XFlush.html for both these functions.