Ristretto is a short shot of espresso coffee made with the normal amount of ground coffee but extracted with about half the amount of water. Traditionally, a ristretto is extracted using the same amount of coffee grounds and water as a normale, but reducing the time to brew it. The result is a concentrated espresso with a different flavor and taste profile, and slightly lower caffeine content.
The brew group is the entire assembly that circulates the water to the coffee puck during the extraction. The brewing group includes only the components after the boiler. A brew group contains as a minimum the group head, and the portafilter, but sometimes has more components depending on the design, or the level of automation. Some brew groups are actively heated, some are passively heated by the boiler through contact. The entire brew group should be sufficiently heated in order to brew a proper espresso.