Scout

Protoss Scouts are medium-sized space superiority fighters; the name "Scout" is a misnomer, as scouting is just one of their roles. Scouts enter the combat theatre through the Stargate, and require no additional buildings, unlike the Carrier. The Scout fires a pair of anti-matter missiles as its primary anti-air attack, which inflict a base 28 points of explosive damage - more powerful than the batteries of a Terran Battlecruiser. While the Scout can also fire dual photon blasters as an anti-ground attack, it is largely ineffective at its 8 damage considering the relative cost of a single Scout. Scouts can be upgraded for better armor and weapons at the Cybernetics Core, better shields at the Forge, and better speed and sight range at the Fleet Beacon.

Upgrades

 * Air Weapons
 * Air Armor
 * Plasma Shields

Apial Sensors
Increases Scout sight to 10.
 * Cost: 100 0 100
 * Upgraded at: Fleet Beacon

Gravitic Thrusters
Increases Scout speed.
 * Cost: 200 0 200
 * Upgraded at: Fleet Beacon

Scout Combat Strategies
As one of seven Protoss units capable of air to air combat, the Scout is perhaps the best (aside from the Archon being effective against Mutalisks). With air to air missiles which inflict up to 28 points of damage, the Scout is a deadly unit. A few Scouts can destroy expensive enemy units such as Battlecruisers and Carriers within seconds. Because of this it is a good idea to keep a few scouts around for mobile defense against air based enemies. Adding to the Scout's versatility is a air to ground attack which does 8 damage. While this may be a weak attack, it is good for a stray unit Hydralisk or two that's giving the Scout a problem. Despite this, never depend on Scouts for an attack against ground units (except for ground units that cannot attack the scout, such as Siege Tanks and Reavers, or for harassing mineral lines) as these expensive units will quickly be destroyed. For this reason, its a good idea to keep Scouts away from groups of ground units as often as possible.

Generally a group of Scouts is safe on its own without backup from other units. However, this is assuming that the group of Scouts isn't engaging any large groups of ground units.

About 36 scouts can handle anything, from air to ground. It is known to kill buildings and units easily by then because there are so many, and there is a small cool down time between each attack.

Obsoleteness
Scouts in Starcraft: Broodwar are are rarely used in competitive play. The Corsair is widely seen as the superior air unit, with numerous more advantages over the Scout. The Corsair's cheap cost, fast build time, splash damage, high speed (without the need for an upgrade), and Disruption Web, give it numerous advantages over the Scout. Because of this, the Scout is rarely built in favor over the Corsair.

Scout Combat Advantages

 * Scouts deal high air-to-air damage with a low cooldown time, making them ideal for assaulting aerial units.
 * Scouts are naturally fast, and upon being upgraded become even faster.
 * Scouts can attack both aerial and ground-based units.

Scout Combat Disadvantages

 * Scouts possess very weak air-to-ground attack, leaving them vulnerable to ground-to-air defenses.
 * Scouts are more expensive than Wraiths or Mutalisks, and take longer to build.
 * Scouts possess no special abilities.

Scouts in StarCraft History
The Praetors Mojo in StarCraft: Enslavers and Praetor Artanis in StarCraft: Brood War pilot Scouts.

Quotes
Praemonitus, promunitus. "Forewarned, forearmed." - Motto of the Venatir Tribe Scouts.