The Zerg are mainly played in games where rushing is allowed.  This is because the Zerg can be produced very quickly.  However, they can also be killed very quickly.  The Queen's Spawn Broodlings ability can turn a single biological enemy unit into two Broodlings.  The Defiler's Plague will reduce any enemy's unit down to a minimum of 1 HP in it's splash range.  If you are playing a no-rush game, then this is generally not the species of choice.  Mutalisks can morph into Guardians.  Guardians are long-range bombers and can attack Terran Turrets or Protoss Cannons while out of their range.  Click on some of the links below to find more detailed information on the Zerg.