The Zerg are a race of purely biological aliens whose main drive is to better themselves and dominate the galaxy by infesting and consuming other species and worlds.  They accomplish this goal by creating highly effective killers and a sheer force of numbers. The Zerg have no mind of their own, there is no independent thought that guides them. Instead they are all under the control of a single entity, the Queen of Blades. But of course, this wasn't always the case - in the original Starcraft, the Queen of Blades succeeded the Overmind.

How They Work
Any Zerg base works as a sort of organism. Even the buildings that the Zerg construct look like organs. This concept is best portrayed by the fact that in order to build a Zerg structure you must sacrifice one of your Drones.

Drones are the resource gatherers of the Zerg, and in order to build any structure you need to command a Drone to morph into the said structure.

Strength In Numbers
You might think this puts some strain on the economy of the Zerg but that is not true. One of the characteristic features of the Zerg is that they can spawn large numbers of their units extremely fast and very cheaply. This basically means that you will never lack enough Drones on your mineral field or won't have a particular building you need for a tech build.
This approach also applies to any of their other units, mainly attack units, which you can quickly amass in large numbers.

This organism-like approach to their evolution does have some drawbacks though. The first and most apparent is that they don't use any sort of technology in order to enhance their traits. The other one is that they can only expand on ground that has been covered by Creep.
This biological substance can be compared to human blood in function, as it is crucial to sustaining the colony.

New Zerg Units and Abilities
Being of such crucial importance to the spread of the colony, two units in Starcraft 2 are capable of spreading the Creep: both the Queen and the Overlord. These two units are able to create small patches of Creep allowing the Zerg to expand at an ever faster rate than in the original game. However in some cases, when combined with the new Nydus Worm unit, this can create entire new strategies.

The Nydus Worm allows your units to fast travel from one part of the Nydus Network to the Nydus Worm's mouth. As long as the destination is covered with Creep this can lead to some very interesting surprise attacks from directions your enemy wouldn't be suspecting.
In the original game you could very effectively spy on your enemies with the help of a Parasite. The SC2 Zerg have taken that ability up a notch and now allow you to effectively take control of an enemy unit and use it against that player. The new Infestor unit allows for this, and it can take control of enemy works as well, giving the Zerg the ability to construct the buildings of other races.

Jase Sherrock