StarCraft The Protoss
High Templars are a special type of unit. While very weak in a fight, Templars posses powerful psionic abilities that make them a powerful weapon. For offense, Templars have the psionic storm. It's an excellent weapon to take out groups of massed enemies - a horde of Zerglings, for example. It is helpful to have a Templar in your group when you enter a Zerg camp. If Hydralisks pop-up to meet your party, blast them with a psionic storm. For defense, Templars have the hallucination power. This makes a copy of the unit you use hallucination on. While the copy is not a real unit, it will draw fire from enemy forces. The final ability requires two Templars - they can join together to form an Archon...
The Archon is a very powerful ground unit which is equally effective against air units. This unit is worth its high cost, but be weary of putting all of your eggs in one basket. Use them as a complement to your Zealots and Dragoons. They will pound the enemy while your other units will help draw some of the return fire.
Scouts are the backbone of the Protoss fleet. While a very capable fighter, it is expensive to build. Instead of trying to produce the resources to build large fleets of scouts, use them to complement your other forces. Their speed makes them excellent flanking units - while you ground forces face the enemy, have a few Scouts hit the enemy from the side. They are also excellent for base defense, able to quickly move to face enemy incursions from any direction.
The Carrier is a powerful ship - even though it doesn't have any built-in weapons. Carriers carry interceptors which can be launched against either air or ground units. A pair of Carriers can unleash a deadly swarm of interceptors that can quickly overwhelm an enemy force. Enemy units will try to shoot the interceptors, but they are so fast they won't take much damage. Your carriers are not invulnerable, though. Scourges and Battlecruisers can make quick work of Carriers, so make sure you keep them protected.
Arbiters make fairly good air units, but it is their unique capabilities that make them especially useful. The first is their ability to cloak. Add one to each of your fleets to mask their true size. Recall is another of the Arbiter's capabilities. It allows you to teleport units to the action instead of ferrying them by shuttle. You can also use this capability to defend a large or several small bases with a smaller force, moving the defenders around as needed. Finally, Arbiters have a stasis field. This will freeze enemy units for a short time. Use a stasis field to remove part of an enemy force from the action until you are ready to deal with it.