Now that you're fighting with the Zerg, things have changed quite a bit. First, the Zerg are a different kind of species, relying heavily on mass quantities of cheap units rather then a few superpowerful weapons. This means you must change your strategic attitudes to succeed with the Zerg.

Another important difference is that all Zerg units are biological, and as such all of them will heal themselves over time. Even a unit or structure one point from destruction can recover given time enough to recuperate. The Zerg also have a different way of building units. Rather then training or manufacturing a unit, they mutate Larvae (always squirming around the Hatchery, Lair, and Hive) into the unit they want. Drones serve a double purpose: They harvest minerals and Vesepene Gas and they can mutate into structures. To "build" a structure, you must have a Drone to spare, because it becomes the structure and is lost forever.

The hierarchy of Zerg minions relates to the Overlords. Like the Terran Supply Depots and the Protoss Pylons, you need the Overlords before you can have other units in action. Each Overlord can be responsible for as many as seven units. If all your Overlords are occupied, you'll have to build more. Once you get the hang of the Zerg, you'll learn that, even though they're not as technologically advanced as the Terran or Protoss, they're still a powerful force.

Read about the The Great Experiment with the Xel`Naga.