6pool

6pool is a Starcraft II zerg rush strategy. It is so named because the strategy centers on building a spawning pool (the "pool") at 6 command, which is to say, before building any additional workers.

6pool build order
The 6pool build order is as follows:

1) Set your drones to harvest initially. Set your hatchery's worker spawn point to a close mineral deposit, and your unit spawn point to the enemy base. 2) At 200 minerals, build a spawning pool. 3) Build a drone as soon as possible, at 50 minerals. 4) Build an overlord as soon as possible, at 100 minerals. 5) The spawning pool should finish production when you are at roughly 100 minerals, allowing you to produce two sets of zerglings. 6) Produce another set of zerglings as soon as you have another 50 minerals, and continue to produce zerglings as often as possible thereafter. 7) With your zerglings, immediately move to attack whatever is necessary. Typically speaking, your best bet is to destroy enemy workers, but sometimes destroying something else, such as a pylon powering a gateway or warping-in photon cannons, can be a better strategy. Your goal should be to destroy all the enemy workers, crippling their economy and winning you the game.

Terrans
Terrans have the easiest time beating 6pool, as typically speaking they should do it anyway; all they have to do is wall off, repair their structures as they are damaged, and they will repel the attack.

Protoss
Protoss have a more difficult time beating 6pool. Their best bet is to wall off as well, either walling off entirely, or walling off enough so that they only have a single building space left open, and place zealots in that space to prevent the zerg from getting through. A forge plus photon cannons can completely stop a 6pool rush, but if the zerglings can get at a vital pylon it can easily end the game if the cannons haven't warped in fast enough.

Zerg
Zerg can counter 6pool rushing by building spine crawlers right next to their hatchery and producing zerglings of their own. It is usually not possible to wall off a 6pool rush as zerg.

Map variations
Typically speaking, 6pool is a bad strategy on any map with more starting locations than there are players; this is because if a 6pool rush misses its target, oftentimes the target will have enough time to establish a wall or build sufficient units or defensive structures to ward off the attack.

6pool is a particularly strong strategy on the Scrap Station map due to the wider base entrances on that map; it is very difficult to completely wall off the ramp before the rush hits, requiring a fall back to faster unit or defensive structure production to stop the attack.