Overview
The Terrans are primarily a species built for a strong defense while the Protoss are built for superior power. If you hope to defeat an enemy with an edge defensively, you'll need to overpower them offensively. If you fail to keep the pressure on a Terran opponent, you'll often find yourself facing well fortified enemy expansions and bases that are difficult to break through and if they bring that defensive front to your front door, preventing you from expanding or scouting, all the superior power at your disposal will not be able to overcome a disadvantage in resource production.

Zealot Rush vs. Marine Rush
As is typical in most games, the first few minutes of the game will most likely involve rushes. The Protoss Zealots can easily overpower Terran Marines (they can take two Marines for every one Zealot) but with the cost and build time difference between the two units, Zealots have to be wary of being vastly outnumbered. When engaging, try and avoid group selecting all of your Zealots and commanding to attack a single Marine since that will most likely result in your Zealots wandering around, trying to circle around each other to reach the target while the other enemy Marines get their free hits in from a distance. Instead, make ample use of the Attack-Move command to have your Zealots quickly attack the nearest enemy Marine.

If you choose to invade with a Zealot Rush, keep an eye out for Bunkers filled with Marines. Once in Bunkers, it is the Marines that hold the advantage with lesser numbers. If you wish to directly confront the Bunkers, make sure you bring enough Zealots (about 2 Zealots for every Marine within a Bunker) to quickly take it down and then kill the Marines within. If your Terran foe happens to build Bunkers away from their actual mining operation, use the Bunker's static position against it by walking past, ignoring the damage taken along the way, and directly attack the mining SCVs. With this type of movement, make sure you are only using the Move Command and not the Attack-Move as you head into the Terran base or else you may find your Zealots being torn apart by Marine fire while they attempt to pound on the Bunkers. In the case that your opponent has built their Bunkers close to their Command Center and SCVs, the best option is to pull back and wait until you have enough Zealots or more advanced units to overwhelm the enemy.

Sneak Attack
One major Terran weakness is the lack of mobile dectection early in the game. If you can get Dark Templar early enough, you should be able to easily walk into a Terran base and quickly raze the buildings and destroy the units. There are only two things to be wary of when you use an early Dark Templar attack, and they are the ComSat Station Scanner Sweep (which will reveal the Dark Templar within the scanned area for a short time) and the Missile Turret (which provide static dectection, much like the Photon Cannons except they lack the ability to hit ground targets).

The Scanner Sweep has a limited detection range and can only be used when the ComSat Station has built up enough energy. If this is all the detection the Terran has when you invade with your Dark Templar, try and force the use of the Scanner Sweep by making a quick single hit and run with a Dark Templar. Hopefully this will cause the Terran to use the Scanner Sweep over their base, but with your Dark Templar well out of the way, revealing nothing. Try this a few more times until you're sure the energy reserves in the ComSat Station have been depleted then walk in and quickly destroy the ComSat Station and then proceed to eliminate your foe.

With Missile Turrets, you may need to take a little more caution, particularly if there are more than one around. Like the Zealots against Bunkers, you can attempt to quickly make it past the detection abilities of the Missile Turrets, however, if Bunkers filled with Marines are nearby you may want to retreat and wait for a later opportunity to use your Dark Templar. If there are only one or two Missile Turrets guarding an area, and if you have enough Dark Tempar, quickly destroy the detectors and once again you will be protected in your cloaking abilities as you proceed to raze the enemy.

In the chance that the Terran has both Missile Turrets and ComSat defending their base, you probably will not want to just walk your Dark Templar into the base. Instead, look for any gaps in the anti-air defense and proceed with a drop from behind with a Shuttle and Dark Templar to put a stop to the enemy resource production.

Handling Siege Tanks
The most potent threat from the Terrans comes in the form of the Terran Siege Tank. These awesome weapons have a staggering attack range and if there are enough of them around, they can quell almost any Protoss ground force. Your best bet is probably with Zealots that have been upgraded with Leg Enhancements (speed upgrade). Since the Siege Tanks cannot fire right next to themselves when in Siege Mode, the Zealot can attack with impunity if they can make it up to the Tanks. If there are several Tanks spread apart, split your Zealot forces into smaller groups and send each group to a different Siege Tank. Not only will this help speed up the process of destroying the Tanks, but also with multiple threats, the Tanks can be "tricked" into firing at the opposing Zealots, causing massive damage against each other (due to their explosive damage type and Large size).

However, those tactics will generally only work against Siege Tanks out in the field lightly defended by Marines or Firebats, etc. If your opponent has built up a substantial blockade using Tanks to support Bunkers and Missile Turrets, then the situation becomes much more complex. Your best bet will be to overwhelm the blockade with 4+ Carriers (more if there are Goliaths or air unit support nearby). Target any Missile Turrets first and chase away any nearby Science Vessels, and you can give any Dark Templar you have a chance to destroy the ground base blockade. Avoid using High Templar and Psionic Storm against this kind of fortified defense, since not only will the Siege Tank be able to fire at the High Templar before it can use Psionic Storm, the Storm will also be useless against the Bunkers and Missile Turrets.

In the latter game, against blockades, look for holes in the base defense and use an Offensive Recall to quickly overwhelm. This will give your ground units the advantage of being able to reach the Siege Tanks without suffering the damage of a normal ground based approach and it can also give you quick access to whatever the blockade is protecting, whether it be a main base or an expansion.

Science Vessels
The Terran Science Vessels are probably the greatest threat for Protoss players. Not only do they serve as mobile detectors, but they also have three key special abilities that can cause massive disarray: EMP Shockwave, Irradiate and Defensive Matrix. EMP Shockwave in particular can be deadly for the Protoss since not only is the energy drained from key units like the High Templar, Arbiter and Dark Archon, it also instantly depletes the Plasma Shields from every Protoss unit and building within its range of effect. As a Protoss player, you'll have to keep a sharp eye for them and hunt them down with Scouts, Corsairs or any other means possible when you see them.

Battlecruisers
The Terran Battlecruiser is probably the closest thing to an "ultimate" unit as you can find in StarCraft, and as such you should always keep your eye out for amassing Battlecruiser fleets. There are several ways you can handle them and they do require a lot of micromanagement as well as your complete attention when facing them. First of all, they are slow moving and as such, are good targets for Psionic Storms. While Psionic Storm will not destroy them in a single use, it will help reduce their massive amount of HP. Once Psionic Storm is cast, quickly follow up with anti-air units (Carriers will probably be your best bet) and keep the pressure hard on them. If you give the Battlecruisers the opportunity to return to a base or expansion to be repaired, you are essentially wasting your time against them. You may also want to use Hallucinated Carriers or Arbiters to help throw the Battlecruisers off balance while keeping the heat from your real units and when attacking be sure to concentrate all firepower on one Battlecruiser at a time. This will help you reduce their numbers faster and by killing them quickly, you will find yourself with less to worry about later in the game.

However, if you know your Terran opponent is going to build mass Battlecruisers, your best bet is to get as many Dark Archons as possible and research Mind Control. By using Mind Control, you can gain the awesome power of the Battlecruiser for your own use and if you have enough Dark Archons at hand when they attack, you can capture an entire fleet within the span of a few heartbeats.