cin.ignore discards characters, up to the number specified, or until the delimiter is reached (if included). If you call it with no arguments, it discards one character from the input buffer.
For example, cin.ignore (80, '\n') would ignore either 80 characters, or as many as it finds until it hits a newline.
cin.sync discards all unread characters from the input buffer. However, it is not guaranteed to do so in each implementation. Therefore, ignore is a better choice if you want consistency.
cin.sync() would just clear out what’s left. The only use I can think of for sync() that can’t be done with ignore is a replacement for system ("PAUSE");:
cin.sync(); //discard unread characters (0 if none) cin.get(); //wait for input
With cin.ignore() and cin.get(), this could be a bit of a mixture:
cin.ignore (std::numeric_limits<std::streamsize>::max(),'\n'); //wait for newline //cin.get()
If there was a newline left over, just putting ignore will seem to skip it. However, putting both will wait for two inputs if there is no newline. Discarding anything that’s not read solves that problem, but again, isn’t consistent.