Scan Converters and Scalers are devices which change the format the signal is represented with.
In detail, the Scan Converters convert a PC signal in a TV signal, or a Composit or Component one.
The Scaler works the opposite way, meaning it converts a Composit or Component signal into a HiRes RGBHV or Y, Pb, Pr one.
Quality in this kind of devices is measured by their ability to make conversions without adding artifacts to the processed signal.