StarCraft The Zerg
If you are playing as the Zerg, you must think like the Zerg. You are basically insects - swarming, fierce, hungry, nasty insects. You must chew-up all the available resources you can find and generate large numbers of units to overwhlem your opposition. But should you just build any units you can and send them blindly into the fray? No, you may be a bug, but you'll need to be a smart bug.
Let's start by looking at the ground units available. First we have the Zergling. Small and cheap to create, they are easy to create en masse. They are useful for emergency base defense; you can quickly have a small army ready to pounce on the invaders. They are also good for swarming larger, less mobile targets. They can quickly overwhelm these types of units before they get a chance to pick off all of your Zerglings one by one. Their main weakness is their vulnerability to air attack. To compensate for this, keep a few Hydralisks with your marauding hordes of Zerglings at all times.
The heart of your forces is the Hydralisk. They are strong units which are effective against both ground and air enemies. They're ranged attack is very useful; place a few of them behind a group of Zerglings. The Zerglings will draw the enemy's fire, leaving your Hydralisks free to attack from a safe distance. They also make excellent escorts for units which are vulnerable to air attack, such as Zerglings and Guardians.
Ultralisks are enormous, well-protected, and can dish-out a lot of punishment. They are also very expensive to produce and are vulnerable to air and swarming attacks. For these reasons, never send them out into the fray without air protection - Hydralisks will do quite nicely. Do not create these units until you are resource-rich. When you do, use them as base destroyers since they can quickly pulverize enemy structures.
This brings us to the Defiler. The Defiler has several uses. The first is to use it as an ambusher. Place Defilers along your enemy's more traveled routes and have them burrow in. When a unit wanders by, they can quickly attack from their hiding places and then return back underground before the enemy can react. Second, they make good hit-and-run attackers against bases. Send them in to gradually weaken a base's defenses. They can strike at the base's perimeter, then retreat and burrow underground while waiting to regenerate. Finally, they can provide air cover for your units with their Deadly Swarm ability. Defilers also have the special ability to regenerate by eating other Zergs. Keep a few Zerglings around as tasty snacks to ensure that your Defilers have the energy to maintain a Deadly Swarm.