A portafilter is the metal handle and basket assembly that holds ground coffee in an espresso machine. You fill it with coffee, lock it into the machine’s group head, and water is forced through it under pressure to extract espresso.
The parts
The portafilter has three main parts. The handle, which you grip. The body, which holds and protects the basket. The basket, the small perforated metal cup where the coffee actually sits during the brew.
Most portafilters use a 58mm basket diameter, which has been the commercial standard for decades. Some entry-level home machines use 51mm or 54mm, which limits your basket and accessory choices.
Spouted vs bottomless
A spouted portafilter has one or two metal spouts directing the espresso into your cup. A bottomless (or naked) portafilter has no spouts; you can see the bottom of the basket directly during the brew.
Bottomless portafilters are diagnostic tools. They show you exactly how the espresso is coming through the puck. If you see one corner gushing earlier than the rest (channeling), you know your prep is off. They also produce a slightly different mouthfeel because the espresso does not contact the metal spouts on its way out.
Pressurized vs unpressurized baskets
Many entry-level machines come with pressurized baskets. These have a small valve that creates artificial pressure regardless of your grind quality. They produce decent crema with terrible coffee, which is exactly the problem; they hide brewing errors instead of revealing them.
Unpressurized (single wall) baskets give you no help. You have to dial in your grind, dose, and tamp properly, but you are rewarded with real espresso. If your machine came with a pressurized basket, switching to unpressurized is the most impactful upgrade you can make for $20.
Care
Rinse the portafilter and basket after every shot. Wipe the basket dry. Once a week, soak in espresso machine cleaner to remove built-up oils. A neglected portafilter develops rancid oils that ruin your shots.