I'll split this post up into three parts, one focusing on each race. I don't want to make it too long otherwise people will complain.
General Overview
I view the Protoss army as very durable and powerful when fighting together. We've all heard of the "deathball" of Protoss units which include stalkers, colossi, sentries, and various other support units depending on the situation. Whenever a Protoss army "wins" an engagement, it usually involves a lopsided battle in which the units lost is very disproportional between the fighting armies. In such engagements, many times you'll see a Zerg army simply melt while the Protoss loses maybe 6 stalkers. And to a lesser extent against Terran bioballs as well.
No this doesn't always happen but it happens much more often than the other way around. The question is how can this army strength disparity be negated. And I think the answer here is economy. Unlike Terrans and Zergs, Protoss macro is more or less consistent, linear growth until it levels off at saturation. Terrans have the MULE mechanic to grow and sustain large armies off of a low base count. Zergs have the luxury of aggressively expanding and taking advantage of pure drone production off of larvae injects. Additionally, reactors and larvae injects allow Terrans and Zergs to remacro their armies very quickly after engagements.
Zerg is on the opposite end of the spectrum when compared to Protoss. Zerg units are rather "squishy" and cheap, thus easily massed. In this sense, army trades are very favorable to the Zerg player. The speed and mobility of the Zerg army (a lot of which is gained through map control) allows for flanks and surrounds which exponentially increases their effectiveness and ability to whittle down the opponent. While squishy, a lot of units in the Zerg arsenal are very deadly and can cripple the enemy if properly used. I'm talking specifically about the baneling though this applies to other units as well in certain favorable unit compositions.
Terrans are kind of an in between race which can either play like a Zerg if opted to go for pure MMM or like a Protoss if going mech. Usually you'll see something in between which leads to a very interesting and dynamic race. It's difficult to completely trade armies against a Terran (similar to Protoss) but when you do, there is usually another remacro'd army waiting back home (like Zerg). Depending on the composition, Terrans have varying degrees of ability to recover from losses as well as goals when engaging an army. While Zergs like to trade armies, if the Terran manages to take out a base while trading, the Terran gains the upperhand in the exchange. Against Protoss, if the Terran takes out the colossi and a good chunk of the Protoss ball, the reinforcing Terran army will be more than enough to press on and continue to apply pressure.
Specific Protoss Mechanics
What makes Protoss such a durable race? The units themselves are very powerful: they deal a large amount of damage and have high HP. But that's not all. Many of them have unique abilities which allow them to live and fight another day.
Forcefields
![[image loading]](http://www.yourstarcraft2guide.com/images/sentryblocking.jpg)
The bane of many players, forcefield is an incredibly powerful spell which can turn the tide of battles. The offensive use of forcefield is clear. They can be used to cut off reinforcements which diminishes the strength of the opponent significantly. They can be used to separate enemy forces and negate melee attacks entirely.This allows the Protoss army to fight small chunks of the opposing armies separately. Reducing the enemy's army size leads to an exponential increase in the efficiency of your army.
Defensively, it can be used to retreat during unfavorable engagements. It serves as an artificial simcity which again decreases the effectiveness of an enemy attack.
Shield Regeneration
After a short period of time, the shields of a damaged Protoss unit quickly regenerates to full health. This quick regeneration is much faster than the regeneration of Zerg units and unlike Terran medivacs and repair, it applies to all units automatically at the same time. Combined with forcefields, it allows Protoss armies to pull back for a few seconds and fight again at full health. After a few seconds, the endurance of a Protoss army essentially doubles (ideally of course).
Blink
![[image loading]](http://starcraft2.danomatic.com/wp-content/uploads/2010/09/sc2_blink_stalker2.jpg)
Combined with rapid shield regeneration, blink allows damaged stalkers to fight from afar, healing up while you let full shield stalkers take the damage. Your DPS stays the same yet the damage you take is very minimal despite similar army strengths. It is also a good defensive ability to protect against harassment (allows Protoss to protect their base and economy). And again, on the retreat it allows the Protoss army to get away with little to no harm.
Colossi
![[image loading]](http://www.sc2-strategy.com/wp-content/uploads/2010/06/colossus-thumb1.jpg)
Cliffwalking and the ability to walk over units gives the colossus very high mobility (even it's speed isn't that slow). Combined with it's powerful attack which can deal damage even at long distances, the colossus is very hardy and requires special, specific units to kill. Engaging a colossus based army without vikings or corruptors is practically suicide.
Tanks
![[image loading]](http://eu.starcraft2.com/features/protoss/images/immortal-thumb2.jpg)
Zealots and immortals are excellent tanky units which can soak up a ton of damage from high damage units. Marauders do very little damage against zealots and siege tanks can't touch immortals. A properly organized Protoss army with tanks in the front, stalkers/colossi in the back, and good forcefields is a terrifying sight.
Warp-in Mechanic
![[image loading]](http://www.sc2sea.com/images/guide/cheese/C%20ZVP%207rr3.jpg)
Among many other benefits, the warp-in mechanic allows a Protoss player to get units anywhere almost instantly wherever needed. But another important and crucial aspect of the mechanic is that it keeps the Protoss army together. Unlike a Zerg player who must rally his units from 5 different hatcheries spread across the map, warp-ins allow small standing armies to instantly reinforce at the correct location.
The Ball
![[image loading]](http://www.teamliquid.net/staff/Plexa//PvZ/pvz2.jpg)
Many features of each of the units allow Protoss balls to be so easily formed. Zealots and sentries have their small size. Colossi are able to walk over units, effectively taking up 0 space. Voidrays/Phoenix are widely used air units and also clump up over your army. You can read about the benefits of having a deathball elsewhere. I'm sure there are plenty of discussions out there about that

Mothership
You don't really see this unit out often, but I think it's interesting how it's abilities complement that of the already super durable Protoss army. Vortex is like a giant stasis (Starcraft I reference) which plays a similar role to forcefields. And the cloaking ability is like the icing on the cake. It makes your units 100% invulnerable without detection.
Moral of the Story
Keep your army alive. Remax and 1a another day.

TLDR
For Dummies




