"Makarov" (technically pronounced "ma-KAR-ov", but everyone I've encountered says it either "MACK-uh-rov" or "mahk-a-rov") is the name commonly used for the Russian "Pistolet Makarova," a small caliber handgun
It fires ammunition in the calibers of .380ACP and 9x18 Makarov (not interchangeably, there are two models... though all that's needed for a conversion is a barrel swap).
9x18 ammunition is not one of the more common calibers, and its name is actually slightly misleading. It actually measures 9.2mm wide and 18mm long, and is slightly more powerful than the more common .380ACP which it so closely resembles. The reason for this odd caliber was the Soviet frame of mind in the time that it was designed.
Just after WW2, the Soviet government decided to replace the Tokarev service pistol , and the Makarov design came out on top. During the war, several facilities such as depots and ammo dumps and the like were captured by the Germans, who actually used the Russian weapons against them once they'd been captured. To make this less likely, they came up with a cartridge unlike any other, the 9x18 Makarov.
Makarovs are, like the AK-47, very reliable guns. Externally they resemble a Walther PPK, but only contain about half as many parts. They operate on a straight blowback design, and recoil is handled by a 17# spring (and your wrist, though it's low). The are of 8+1 round capacity, with a 10/12+1 round Russian-made version available.
Maks are fairly common for a Russian-surplus firearm because they are very cheap and quite small, but have the aforementioned quality in spades. They are quite easy to carry concealed (not recommended unless you are licensed to do so).
Several communist-block nations produced Maks, and there was a version made in East Germany, Bulgaria, Russia, and China. The East German is supposed to be the nicest of the lot, but are comparitively rare (which classifies them as Curios and Relics, technically). The Bulgarians have excellent fit and finish from the factory. The Russians can be spotted because they are the only ones to feature adjustable sights, and were the only ones to be available in the high-capacity double-stack version. The Chinese Maks are very rare, so they command a premium, though I am unsure of the quality.
All in all, if you are in the market for a sub-$200 semi-automatic of high quality, you could definitely do a lot worse than the Mak.