Strictly speaking, percolators are a sub-category of filter coffee machines. Water is heated, ascends as steam, wets the coffee powder and drips through a metal sieve into the collecting container. There are three types of percolators:
The water rises from the container through an inner tube, wets the coffee powder and is collected in the same container again.
In contrast to circulating percolators, pumping percolators have an additional container to collect the water having passed the coffee powder as finished coffee. The working principle is basically identical to that of a filter coffee machine.
Probably the most popular of these is the Bialetti moka pot. Water is heated to boiling tempearture in a pressure-tight system (moka pot). The pressurized steam directly penetrates the coffee powder, then rises through a tube to an upper chamber, where the coffee is collected.
Mistakenly, the coffee prepared in this moka pot is often referred to as espresso. Since percolators only use low or no extra pressure - and the typical espresso crema requires 9 bar to form - the final product is still a moka.