These are protoss versus zerg (PvZ) strategies used in StarCraft and StarCraft: Brood War matches.
Overview
Protoss players frequently play defensively against zerg. Zerg are more mobile, and may counter early protoss units at even cost with just zerglings. Protoss must rely on higher tech to effectively attack - templar, corsairs, archons, reavers, or dark templar; photon cannons and zealots are used in the interim. A challenge is to choose a strategy that is robust against the zerg build; the protoss may not have time to switch strategy and tech.
Once the zerg contain is broken, the focus shifts to attacking zerg expansions and their economy. The zerg require large amounts of vespene gas, making it more effective to strike expansions rather than the main base. If the zerg can be prevented from holding three or more geysers, the protoss may be able to achieve victory through containment and attrition.
Like in matches against terrans, the protoss do not rely heavily on expansion. The protoss army is too slow to defend distant expansions against zerglings and mutalisks.
These factors mean PvZ matches tend to be shorter. The mid-game is expedited by protoss teching, and the protoss focus on containment rather than expansion means the late-game may happen before all minerals are mined.
Common Early Game
2-Gate Zealot Rush
Construct a pylon and two gateways at 9/10. Build another two gateways at 12/18, followed by mass production of zealots for a rush.
Natural Forge/cannon to early expand
The Bisu build. This is an economic, fast-expand build. Build an 8-pylon->forge->2 cannons to hold your natural and your ramp. Then build a 14-nexus followed by gateway.
Because it takes fewer minerals for zerglings to destroy photon cannons than the cannons themselves cost, this is dangerous, and should only be used on maps that enable you to defend your natural and ramp with the cannons. It's imperative that you build defense before your nexus, because Protoss will have a time disadvantage. Otherwise a quick 5-pool or 9-pool and 6 zerglings can easily get into your base and wreak havoc before you get a single cannon in.
Fast Stargate
This is often the build espoused by most Protoss vs Zerg matches where the Zerg has managed (as is common) to contain the protoss. Corsairs, available the moment the stargate is finished, are used to destroy scouting overlords, and prevents a zerg mutalisk rush. With enough corsairs (6+), they can easily take out any advancing scourges, which leave the zerg no choice but to use hydralisks to defend their overlords. This requires heavy micromanagement, and should be reserved until later in the game, when the protoss economy can afford spending a great deal in a harassing strategy.
Counters A pair of scourges can take out a corsair with minimal minerals, and less cost in gas (75 gas for 2 scourges, vs 100 for a corsair). Both teams require heavy micromanagement to ensure that neither gets an upper hand in a fast air rush.
Rare Early Game
Cannon Rush
Either pylon in opponent's base and forge there followed by cannons or pylon->forge in your base and pylon->cannons in your opponent's base. Seen almost exclusively in low level play, even rare in trick play, because caught early enough it is very tricky to deal with. It takes four zerglings to destroy a cannon, so the protoss player has a 50 mineral disadvantage in this regard. It requires multiple cannons catching the zerg player off guard, where 2 cannons can take out a zergling in one shot and overcome 8 or more zerglings. Particularly effective if you can funnel the zerglings through a chokepoint, or place your cannons in a place that is difficult to reach yet covers the main or natural mineral lines.
Common Mid Game
Corsair Harass
Reaver drop
Dark Templar drop
+1 Zealot Attack
Zealots normally require 3 hits to kill a Zergling, however, if the Protoss player has one greater attack upgrade than the Zerg's carapace upgrades, then the Zealots require only two hits, doubling the effectiveness of the Zealot against Zerglings. Fast forge + mass Zealots early game. Counters An overlord watching over the base can see if a forge is being built, and, if it's spinning, that can only mean a +1 Zealot rush. Building an evolution chamber and matching the attack with a carapace upgrade ensures that Zerglings are economically advantageous. Otherwise, a fast tech to lurkers or mutalisks may also work.
Counters
Lurkers in chokepoints. Mutalisks harassing the opponent while their army is away.
Zealots, Archons, and High Templar
The big protoss push, sometimes called the protoss "big ball". Use high templar to psionic storm lurkers, hydralisks, and mutalisks, and the rest of your "ball" to punch through expansion defenses and ruin zerg's economy. Use archons to also defend against the air. Make sure you have zealot speed and at least +1 attack to give your zealots that extra punch. Add dragoons or reavers if lurkers become a real serious threat (see Full Tech, below).
Counters Lurkers, spread out to require one storm per lurker. Observers are rare at this stage, making it very difficult to deal with lurkers. Lurkers are even better if you use scourge and an overlord to take out observers. Later, ultralisks and zerglings can be very effective, or mass hydralisks. If you can, dodge those storms! Anticipate that the opponent will use their high templar and don't send your units in all at once to get stormed to death. Instead attack in small harassment waves until storms are exhausted. Dark swarm on archons/cannons and plague on the zealots are also very effective. If you have them already, mutalisks are great to pick off the high templar. Alternatively use queens to spawn broodlings on the templar, utilizing their range and wasting none of your infantry. Or, use your lurkers to slow the protoss down and use the rest of your army to harass and distract the protoss' main force while you expand.
Rare Mid Game
Dark templar/corsair combination
In the late game you are no doubt suffering a disadvantage as protoss. It is time to stop using corsairs to simply harrass, but to help you win the game. After you have 11-12 corsairs take out as many overlords in the enemy base as you can. Use the remaining corsairs to elminate any detectors they may have around their troops, and at the same time send in dark templar.
This method is also very effective with additional zealots to assist your dark templar
Common Late Game
Full tech
Zealots, dragoons, archons, reavers, observers, and even templar combine to drive the zerg army back and strike at the zerg's expansions.
Counters Ultralisks can lead a strong ultraling or ultra/hydra charge, supported by defilers. Lurkers can slow this down, but ultimately you'll need a lot of money and expansions to compete.
Corsair/Reaver
This strategy requires heavy micromanagement as the protoss player needs to manage both his main army and his corsair/reavers(in shuttles) team. The corsair/reaver squad utilizes its mobility to hit-and-run on zerg expansions. The reavers' high attack points with its splash ability can quickly decimate the zerg economy if the zerg player does not react fast enough to save his drones. While the corsairs' primary focus is protecting the shuttles from scourges, they can take down any overlords that come into sight, forcing the Zerg player to waste his mineral and limiting his supply cap if enough overlords were killed in quick succession. Protoss players rarely upgrade the Disruption Web ability for corsairs from the Fleet Beacon and cripple masses of hydralisks in a battle, rendering them useless against the reavers. A skilled protoss player may take out the zerg's main army or additional expansions while the corsair/reaver fleet is creating a diversion. Although poor micromanagement can lead to shuttles being destroyed by scourges and reavers by masses of hydralisks, this strategy is one of the most potent tactic used by the protoss. A good example of this strategy is shown in 2007 Ever OSL Semi-Finals Bisu vs sAviOr Set 3 in Katrina.
The use of this strategy is greatly discouraged for in a protoss v protoss or protoss v terran situation as the corsairs will be useless against protoss or terrans. Instead, simply use reavers in a shuttle without escorting them with corsairs. The reavers serve the same primary purpose-to cripple enemy economy.
Another crucial role the reaver plays in the protoss v protoss and protoss v terran matchup is to support the dragoons in battle, using its massive splash damage to quickly weaken the enemy units for the dragoons to take care of.
Counter: A counter for the corsair/reaver is burrowing zerglings all around the map to locate the fleet's movement, and taking them down mid-flight with hydralisks (and preferably a few scourges). If a large amount of corsairs are taken down, the Zerg player may even decide to use mutalisks to harass the protoss since corsairs in small numbers are extremely vulnerable to scourges.
Corsair/Dark Templar
Storm Drop
The storm drop is similar to the reaver drop. High templar attack worker lines with Psionic Storm. The attack is less effective against terran SCVs than drones and probes; the SCV have more hit points to lose when leaving a storm's area of effect. Reaver drops ensure more minimum kills, while the storm drop has the potential to be more cost effective and devastating.
High templar move slowly. Use faster zealots to lure defending troops into the high templar's psi storm range, rather than move the high templar to the enemy. The zealots may also cover the high templar.
Counter: Like the reaver drop, the best defense against the storm drop is to prevent the drop from happening with anti-air structures and units. Terran players may plant spider mines near their workers to catch high templar as they deploy.