Orders of BattleEdit
Expect the opposing terrans to be utilizing the Terran vs. Zerg orders.
4-Pool/5-Pool Zergling RushEdit
A "4-Pool" means you're starting to build a spawning pool using your 4th drone. This means do not purchase a drone with your first mineral coffers, but instead wait to use them for the 200/150 mineral cost for a spawning pool. Once the spawning pool is completed, immediately build 6 zerglings, and send as many drones as you can spare to attack. A "5-Pool" is similar, but does build a drone with the starting 50 minerals. A 5-Pool is only slightly slower, but provides an extra drone to scout or defend with.
The 4-Pool gives the zerg one of the fastest, most vicious, early rushes in the game. However, success depends significantly on speed and the distance between opponents - even more so than most rushes. On large maps, it's critical to know where your opponent is so you can attack the right location as soon as the zerglings are done. (Obviously, this is not a problem in 2v2 on 4-player maps, and so this strategy is common in 2v2 games and can be used against any race. The build is so dangerous because it strikes down a terran before MnM, when marines are most vulnerable.
The build is vulnerable to scouting, but the opponent will have a hard time defending against it if they haven't concentrated on defenses right at the start.
5-Pool has been used effectively in the JJU vs. Reach 2005 match, and more recently in the Ausussum vs. Quaalude 2009 match.
If you suspect a 4-Pool, don't fast expand. StarCraft is a game of many rock-paper-scissors, and it is very tricky to beat a 4-pool while expanding. Instead, try and get your barracks up on 10/11 SCVs at the latest. While 4-pool is so early you can miss it if you scout it last, it gives a critical advantage if successful. If you see the zerglings coming, use SCVs or buildings to block your entrance, and place the marines you have behind. You can also clump your buildings together near your command center and hide your marines in your mineral line, thus making it very hard for the zerglings to attack any of your units or buildings. Don't hesitate to fight back with as many SCVs as you need, or use them as walls for your marines. If you can deflect the 4-pool you will likely have an easy win.
A 9-pool means building a spawning pool immediately after your 9th drone, then an overlord, and then saving minerals and larvae for zerglings. The build is safe and flexible. It provides some early protection, and the number of zerglings can vary depending on what your goal is. 4 zerglings usually are used kill scouts and harassers, playing cheap mobile defense. 6 zerglings are usually used to play aggressively and try and scout and harass your opponenet's base, at the cost of an extra drone. 6 zerglings are usually accompanied with zergling speed, to make escapes and runbys easier.
The 9 pool does, however, is not very strong for production, as compared to an early hatchery. Being stuck on 1 hatchery longer means you can produce fewer drones, and so the 9 pool is usually played aggressively to hurt the opponent's economy as a payoff, while you get your second hatchery right away. The 4-zergling variant is reserved situations where 4 is enough or the scout really needs to be destroyed. Which variant you use is also very map-dependent. 9-pool in general is less useful on a larger macro-oriented map, where it takes longer to travel between bases.
The 9-pool is frequently along the path in other builds.
Counters Fast expanding against a 9 pool should give you the advantage, provided you can hold off the harassing zerglings. To prevent the runby, you can also block your ramp with SCVs and marines.
This is to have a more stable game, but with the risk of losing if the opponent does a 6-barrack or an early rush. The order is as follows (parentheses denotes the number of drones). 9 drones, overlord, hatchery expansion (12), spawning pool (11), gas (10).
If the terran moves out with marines and medics, if you spot it immediately you can try a mass 'speed' zergling attack into their main as the marines get halfway across the map. If you can get into their main, you can usually pick off much of their worker economy before they could return, and use sunkens and new zerglings at your main to hold off the marines and medics.
Position a ling a bit back from their entrance so you can see when they break out, and then a good clump of lings hidden off to the side. About 10-15 seconds after the marines spot your scouting ling and move towards your base, when you suspect the time is right, run your zerglings into the terran base. Getting up the ramp is the tricky part; try and force your way through if you can, but don't overcommit and lose all your lings if it's well covered, save them instead to help take out the attacking marine force.
Be wary of a terran trick with a fake attack and don't hesitate to use a second zergling scout to spot it.
Counters A blocked ramp can easily stop the counterattack. Use newly built marines to block your ramp, at least 3 or 4. 2 firebats are even better at this. If you suspect a runby, have a plan for it before you leave your base - don't attack if you can't stop the zergling counterattack.
You can also fake the marine attack by taking out the scouting zergling but not really pushing out much further. This can not only trick the zerglings into a trap, it can trick the zerg player to waste drones and money on sunken colonies for defense.
A '7-pool is identical to the 5-pool except that a spawning pool is built on the seventh drone instead of the fifth. This strategy has been mathematically shown to be strictly worse than the 5-pool; the latter will always provided more zerglings at a faster rate.
This is a slower version of the 6-muta rush. It forgoes defense and is practical mainly in team games. Start with 9-hatch or 12=hatch and acquire two extractors. Use the first 100 gas to upgrade to lair followed by a spire. Hatch two additional overlords in the meantime.
If the enemy has no anti-air defenses or units, the mutalisks have the advantage for at least 60-90 seconds. The mutalisks may continue to have free reign as the enemy builds up counters.
One useful tactic with mutalisks is to group 11 mutalisks and an overlord (or any unit outside of the screen with mutalisks). This allows the player to pack the mutalisks tightly, which is useful for attacking marines and turrets, and provides ease for micromanagement. A disadvantage with this tactic is the science vessel's Irradiate, which will virtually render this tactic useless.
Similar to the Zerg vs. Protoss Hydra Bust. Very vulnerable to tanks.
In a late game against terran, the hive upgrade is essential, because it allows the player to upgrade Melee and Missile Attacks and Carapace. Furthermore, it allows the player to produce defilers and ultralisks. After a hive upgrade, build a defiler mound first, and upgrade the Consume ability. This allows a defiler to replenish its energy for Dark Swarms and Plagues. Defiler is essential in a ZvT game, especially the Dark Swarm, because it renders all range attacks useless (does not apply to siege tanks due to its splash damage, and lurkers).
A tactic that has grown popular recently is the utilization of queens. Using the Broodling ability, queens can get rid of siege tanks, then the remaining terran units (which will most likely consist of marine/firebat + medic or vulture + goliaths) can be eradicated by a combination of zerglings, hydralisks, lurkers and defilers. The Ensnare ability can be also used on marine/firebat + medic, which decreases their moving speed and attacking speed.
Mass overlord drop of ground units.
Hydra-Ling uses a group of zerglings and a group of hydralisks to counter terran ground forces. The larger group of zerglings pin down the enemy which the hydralisks contribute fire in relative safety. Burrowed zerglings can ambush the enemy and force an engagement; this is particularly useful against siege tanks.