A ristretto is a "restricted" espresso: the same coffee, stopped early so it is about half the volume. It is sweeter, thicker and less bitter, because the last, harsher part of the shot never reaches the cup.
The differences
Volume: a ristretto is about 15–20 ml per single shot; an espresso about 30 ml.
Taste: a ristretto is sweeter and syrupy; an espresso more balanced and a little more bitter.
Caffeine: a ristretto has slightly less, since less water passed through the coffee.
