A portafilter (or group handle) attaches to the group head of semi-automatic and piston-driven espresso machines, and carries a tamped puck of coffee grounds within its basket. It is usually made of brass, and is attached by a plastic or wooden handle. The portafilter forms a seal with the espresso machine's gasket, and directs high-pressure hot water through the coffee puck.
After-market retailers also sell bottomless portafilters that minimize the espresso's contact with any metal. A bottomless portafilter is one tool baristas use to analyze the quality of the coffee grind and the evenness of the extraction. Often baristas use knockboxes to store their spent espresso grounds after they have pulled a shot.
Some home espresso machines come with a pressurized portafilter that is designed to make it easier to brew espresso with crema for novice users. The drawback of these portafilters is that the barista does not have much control over the quality of the shot.