The Makarov PM is a 9x18mm semi-automatic sidearm based on the real world Soviet pistol. While it benefits from a relatively quiet gunshot and abundant ammo (Makarov magazines are found commonly throughout Chernarus, even on dead zombies), it is severely hampered by low accuracy and weak fire power.
Per-shot damage is similar to weapons firing 9x19mm rounds (G17, M9, and PDW), but the Makarov uses 8-round magazines compared to the 15-round magazines of the M9, 17-round magazines of the G17, and the up to 30 rounds of the PDW. This means that Makarov ammunition, while extremely abundant, makes very poor use of inventory space, since each slot can only hold 8 of the Makarov's rounds.
Even so, the Makarov is relatively quiet and like all guns will kill a zombie with a single headshot, so it remains useful especially to players who have yet to find a more suitable weapon. It is also a very effective weapon if you are good at headshotting zombies, as you will never run out of ammo.