The Scout (not to be confused with the Scouts of the Space Marines) is a class from Dungeons and Dragons that is something of a mix btween the Ranger's archery and the Rogue's sneakiness. While most would consider a Scout type class to be rather pointless given that its basic themes can already be handled reasonably well by the Ranger and the Rogue, Dungeons & Dragons has never been shy about putting the spurs to a horse no matter how tired it is, and so there's been a few encounters with in over the editions.
In 2nd edition, Advanced Dungeons & Dragons, the Scout appeared as one of the Kits in the Complete Thieves' Handbook, and was literally nothing more than a "wilderness rogue"; a rogue who gained a boost to mobility related thieving skills in wilderness environments, and had a better chance of launching ambushes, but who suffered a penalty to all their thieving skills in urban environments.
In 3rd edition the Scout was designed around moving fast while shooting at the enemy with deadly aim. It was a complete class in its own right released in the Complete Adventurer, alongside the Ninja and the Spellthief. Think of them as 60% Ranger, 30% Rogue and filled out with equal parts Druid, Barbarian and Monk. They are 3.5e's other class that primarily uses a bow, and they're quite good at it. Though they only have 15/10/5 BAB, one good save (REF), a limited D8 HD and not too wide a weapon selection, they possess a staggering 8+INT skills and can be used as competent melee fighters as well.
The Scout's main class feature is Skirmish: if they move at least 10ft they get a bonus to both their AC and get an extra d6 Precision Damage. These abilities grow as the character levels up: every odd numbered level the damage or the AC go up by 1, taking turns until they max out at 5d6 and +5 AC at level 19. This makes moving around very important in order to deal lots of damage. Remember: the more damage you deal, the quicker a fight will be over and the less damage will be dealt to you. Note that Skirmish is not limited to ranged attacks and that your melee attacks also get the bonus if you can close the distance. The fun thing about Skirmish is that while it deals precision damage (and as such can only be trigged once on a creature per turn), it is not limited by on how many creatures per turn it can be used. Take Greater Manyshot and with a good roll you can deal your precision damage aganst multiple enemies in one shot.
The Scout also gets a large number of class features pilfered from a variety of core classes: Trapfinding, Battle Fortitude, Uncanny Dodge, Fast Movement, Trackless Step, Evasion, Flawless Stride, Camouflage, Blindesense (Blindsight at 20) and Hide In Plain Sight. They also get a permanent Free Movement effect at level 18 and get a bonus feat (from a limited list) every 4 levels. What should be noted is that many of the Scout's class features are limited by their encumberance: if they hit a medium load they immidately lose a good chunk of them including the excellent Skirmish.
Various races can play an effective Scout: Humans work as always, as do Elves. Halflings can pull it off too: while their smaller weapons deal less damage, Skirmish does not scale to size and manages to deal enough damage to make up for the lost damage. The Xeph from the Expanded Psionics Handbook make for an interesting addition as well: they get a bonus to DEX, have a racial ability that speeds them up even more (allowing for 80' movement per turn at level 12 in combination with Fast Movement) and they are psionic to allow for things like Speed of Thought and Up the Walls.
Scouts have a notable list of skills taken from a mix of the Ranger and Rogue lists: they have plenty of stealth and nature skills alongside movement skills and a few odd ones like Sense Motive (perfect for spying) and Disable Device (added via Errata: note that a Scout cannot disable nonmatical traps). Hide, Move Silently, Tumble are the most important ones: Disable Device, Spot, Listen and Search make for good others. Investing in Use Magic Device as a cross-class skill is very useful: not only does it allow interaction with magical items it also allows for using the Sparring Dummy of the Master to turn the Scout's 5' step into a 10' step, making it even easier to trigger Skirmish.
When dealing with magic items, things that increase your Dexterity take the priority, followed by things that increase your mobility, items that allow you to deal more damage, items that increase your skills like Hide and Move Silently and items that protect you against magic.
While you can take the class for a full 20 levels, there is a sweet multiclass feat with Ranger called Swift Hunter that makes a pretty neat build. Its big draw is that you will always apply Skirmish damage to Favored Enemies, so pick undead and/or construct to slap them with that delicious bonus damage.
Pathfinder made them a rogue archetype, with Skirmish replacing Uncanny Dodge. Essentially, this means that the AC bonus is gone, but the scout now gets full Sneak Attack progression, but can't make Sneak Attacks at once.
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|
The 4e Scout is one of the two Essentials Variant Classes for the Ranger, introduced in "Heroes of the Forgotten Kingdoms" as a "simplified" take on the Ranger, built entirely from the Two-Weapon Fighting archetype/subclass of the ranger. Its counterpart was the Archery-based Hunter.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
In 5e the Scout resurfaced as a new Fighter subclass, in the Unearthed Arcana article "Classics Revisted/Kits of Old", in which it was essentially a beefier, non-magic-using take on the Ranger. This incensed many Ranger fans since even with its comparative limitations, it was still a lot better at being a ranger than the actual ranger class.
Post-Ranger rework, the scout was again released as a rogue archetype, that gets the ability to run away as a reaction if an enemy closes in, free proficiency *and* expertise in the Nature and Survival skills, a lot of extra speed, the ability to support your team while ambushing an enemy, and, at level 17, the ability to make an extra attack on a second target as a bonus action. With Sneak Attack.
|Dungeons & Dragons 5th Edition Classes|
| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
|Artificer - Mystic|