A moka pot is what some call a stovetop espresso maker; I find this to be a bit of a misnomer, because the result is rather different than proper espresso. Water is put into a bottom vessel; coffee grounds are placed on a perforated platform with attached cone, which is placed on/in the water vessel; a top holding tank with rubber gasket is screwed on top of the apparatus. The device is placed over a heat source, boiling the water; the boiling water creates steam and pressure, which forces water through the grounds into the upper storage vessel. The two chambers are separate, so the water is forced through the grounds once (upward). Somewhat similar to espresso, a moka pot develops some small amount of pressure to force (nearly boiling) water through grounds, though the pressure is much less than espresso (see also this answer). I don't think the grounds are meant to be tamped like espresso, either.
A percolator boils water and drips it over grounds continuously. There is no separate storage vessel for the coffee once brewed; water (at first) or (partially-brewed) coffee is continuously boiled (passing up to the top of the chamber through a funnel/straw) and dripped over the grounds (which are instead at the top of the chamber, rather than in the middle as in the moka pot). The brewed coffee ends up in the same place the plain water was at the beginning of the brew. Percolators often have a clever little clear cap in the middle of the top, so you can see when the brewing is occurring, and color of the brewed coffee.
I have both devices, and use them differently; e.g., a coarser grind for percolator. I find the outcome of percolated coffee to be more like French press or perhaps drip; moka is more like Turkish coffee or espresso. Though similar in concept (stovetop coffee production devices), they're (to me) totally different in outcome.
I suspect, but I don't know, if there are regional differences in these terms.
Full disclosure: this meta question and my answer prompted this adaptation as a clarification question.