Zerglings are the first offensive units in the game. Besides the SCV, the Zergling is the next best unit, till marines and zealots are produced. It's very easy to rush with these units, since a hatchery is able to produce 3 units at once. This allows the creation of 6 zerglings, and another pair shortly. Zerglings have had a huge impact in many games, especially in small maps, where they can hit their targets quickly.
None of the worker units can match up to the zerglings, except for the SCV which will lose, but has a better chance than the other two workers. This means that a zergling rush against a Protoss, or Zerg player will hurt them severly if they have no units, since they'll lose far more workers than a Terran would.
Zerglings are fast units, and can be produced in pairs. When the early rushing stage is over, they make great scouts. Create a pair or two, and send them to unclaimed expansions. This is very helpful since you would rather lose the zergling than an overlord. Make it a habit to send out zerglings to stand guard at ecery expansion and in front of your opponents main base. Burrowing is a great addition, making the zergling one of the best scouts.
Zerglings are very weak, yet they have a great melee attack. They can deal the a lot of damage in a ver short period of time. They are also deal the most damage for their cost. However small choke points, will slaughter zerglings, since they all must be able to swarm the enemy. Large open maps are ideal.
The speed upgrade is essential if you want your zerglings to engage the enemy faster. It also allows a zergling to scout faster around the map. The adrenal gland upgrade, is a huge addition to th zergling. It basically doubles the attack rate, like stim pack, except that it is permanent. Once zerglings are upgraded with adrenal glands, they become dangerous on open maps, where it's easy to surround an enemy.
Zerglings are also perfect units to combine with dark swarm. Since you get units for the price of one larva, you can take advantage of this and use consume. This will ensure unlimited dark swarms.
