In single flow taps the hot and cold water mix within the tap body and spout, by the time the water has left the spout the water should be well mixed to the desired temperature.
Single flow taps will generally have a better flow rate than an identical dual flow tap, as the outlet diameter is greater.
The down side it that there is an increased possibility of back flow so it is advisable to install non return valves.
In dual flow taps the hot water is fed through a separate pipe within the spout, the cold water passes around the outside of this pipe.
As the water only mixes once it has left the tap this gives the benefit the spout should never become uncomfortably hot.
As both hot and cold are kept separate the chance of backflow is minimised.