A French Press consists of a narrow glass cylinder, equipped with a plastic lid and a "plunger" which fits tightly in the cylinder and has a fine wire or nylon mesh acting as a filter. In Australia the whole apparatus is known as a plunger and coffee brewed in it is known as plunger coffee.
In order to brew coffee in a French Press, one removes the lid and plunger, puts the desired amount of coffee grounds inside the cylinder, pours hot water to almost fill the cylinder, puts the lid and plunger back on, waits between 3 to 5 minutes until the infusion is made, then presses the plunger down, thus separating the coffee grounds at the bottom of the pot. At this point the coffee can be served.
Because the coffee grounds are in direct contact with the brewing water, coffee brewed with the French Press captures more of coffee's flavor and essential oils, which often become trapped in a traditional drip brew machine's paper filters. Thus, to coffee purists, it's one of the preferred brewing methods. However, it's also usually stronger, thicker and has more sediments than drip-brewed coffee.
Coffee for use in a French Press should be of a somewhat fine grind, similar to that used for a drip brew coffee filter, but slightly coarser than that used for espresso. Coarser grinds such as used for coffee percolators give less satisfactory results.
In the UK this device is known by its French name, cafetière, or as a 'plunge filter coffee maker'.