Like the Larvae from which they hatch, Zerg Drones contain within them the genetic code required to morph into any of the large Zerg "buildings", which are actually living (but immobile) structures. They can only grow on areas where the Creep has established itself, as they require its nourishment and building material. The Drones also harvest any raw materials that are needed. Vespene gas is very important to the Zerg: the high energy state of the gas powers their accelerated metabolisms and is essential for the production of the more advanced Zerg breeds.

Using the Burrow Ability
The Zerg Drone has one major advantage over the Protoss Probes or the Terran SCVs, and that is the ability to Burrow. Before a Drone can use this ability, Burrowing must first be evolved at one of the Hatcheries/Lairs/Hives. Since Drones are relatively weak attackers, Burrowing Drones for ambushes is almost never a good idea. However, Burrow can become very useful for the Drones when an enemy force comes pouring in. By Burrowing the Drones, you can give them the chance to survive until reinforcements arrive, or until the enemy has the ability to detect the Burrowed units. Should you repel the attack, you can quickly unburrow all of the Drones and resume resourcing immediately.

Building

The Zerg Drone contains the genetic template to morph into any of the basic Zerg structures. As the colony moves up the evolutionary ladder, the Drone is infused with additional genetic code to morph into more advanced structures.

The key difference between the Zerg method of colony construction and the Terran or Protoss methods is that the Drone actually morphs into the structure. The Drone can be recovered during the morphing process if the structure is canceled, however once the structure is completed, the Drone is permenantly lost. Note, however, that the once the Drone enters into its morphing stage, the control for the Drone is no longer needed and a few interesting strategies have arisen with this particular property.

Resource  Mining

The Drone is capable of mining both minerals and Vespene gas. To gather minerals, just right-click on a mineral cluster with the Drone and it will automatically begin resourcing. Each mineral cluster can only be mined by one Drone at a time. If only one Drone is assigned to a particular cluster, it will mine only from that cluster. However, having only one Drone per mineral cluster isn't the best ratio for mineral gathering -- usually you will want to have at least 2-3 Drones per cluster to get the most out of the minerals (however, depending on the distance of the Hatchery from the clusters, you may want more or less). Note that there can be too many Drones mining at one time and those Drones that are unable to access a Mineral cluster will usually just end up sitting idle until a cluster frees up. With experience you should be able to find optimal numbers for mineral clusters for a particular resource area.

In the case of Vespene gas, an Extractor needs be built on top of the mine before Drones can collect from it. Once the Extractor is completed, just right-click on it with a Drone to begin collecting the refined Vespene gas. The number of Drones you'll want gathering from a mine will depend on the distance between the mine and the destination Hatchery. Note that although only one Drone can be inside the Extractor at a time, 3-4 Drones set to gather from each nearby Extractor will usually offer maximum efficiency. The general rule of thumb is to keep adding Drones to the Vespene gas Extractor until only one Drone waits outside at any given time. If more Drones begin waiting around the Extractor, then those additional Drones provide no improvement in the rate of Vespene gas harvesting.

Upgrades

Burrow - Using the Burrow ability Drones will be able to hide from more powerful enemies.

Carapace - Each upgrade adds +1 to the Carapace for a total of +3 when fully upgraded.

As mentioned earlier the Burrow ability can be extremely helpful to your Drones during an early game invasion -- if you have the resources available, you should evolve it as soon as the chance arises. While the Drone can attack and defend, it is a fairly weak unit and although the Carapace upgrades may help a little, they will rarely make a difference in a battle as far as the Drone is concerned.