A Strategy Game subgenre which focuses on construction and control of a fighting force in battle that takes place in real-time. It has an army of tropes all its own.
Debate rages as to what the first originating RTS actually was, the results largely dependent on the age and nationality of the participants. The ZX Spectrum had some early entrants many many moons ago (1984 saw Stonkers and 1987 — Nether Earth); the MSX saw Herzog in the late 80s — but only in Japan; and 1989 saw its sequel Herzog Zwei on the Sega Genesis (Mega Drive) which introduced many of the genre conventions. But it was 1992's Dune II by Westwood Studios (later of Command & Conquer fame) that established the form and the tropes of the RTS which are known to the masses today.
In comparison to Turn Based Strategy, Real Time Strategy has players take action whenever they wish (within the constraints of how often any particular action can be taken). Play will continue for other players and entities in the game even if one player sits back and does nothing, rather than breaking up the sequence of play into discrete turns for each player, which brings multiplayer matches down from hours or days to minutes. While this leaves less time for strategizing and encourages "twitchy" play-styles, it makes commanders less omniscient, allowing for ambushes and distractions to work better.
Almost all games of this type are based on ground combat on a 2D map, with various terrain and obstacles adding interest to the battlefield. Airborne and seaborne units are sometimes incorporated, but they are seldom realistic in terms of speed and performance (almost all aircraft in RTS games can hover, cannot change altitude, and have unlimited fuel and ammo).
The basic archetypes of units in a typical RTS:
Builder/Harvester — Some games have units entirely dedicated to gathering resources, while another unit handles construction. This is also used if one of those jobs is performed by a building instead, with the Peon handling the other.
The Militia — This is essentially the Peon's supermode (if you can even call it that), with better attack power than a Peon but less than that of a normal basic ground unit. Useful en masse in resisting a rush.
The Basic Melee Unit — In medieval fantasy games this is the first and most basic combat unit available, and most often has a sword and light armor. In more futuristic scenarios, melee units are much rarer.
The Basic Ranged Unit — The first and most basic combat unit available in most modern, futuristic, and historical games set in the age of the gun, the second in medieval fantasy. Has a rifle or a bow or other basic ranged weapon, and pathetic armor.
The Anti-Something Infantry — Usually used in modern/futuristic games. About the same stats as basic units, but usually armed with a grenade or rocket launcher or other specialized weapon. Designed as a specific counter to some other type of unit (usually vehicles/buildings or air units), as opposed to the more versatile basic units.
The Scout — A fast unit with limited combat capabilities and a wide sight range. Used to reduce the Fog of War, though it may also double up as a stealth/cloak detector, light combat unit, or target designator for artillery/superweapons/etc. Can be a light tank, jeep, or motorcycle in modern and future games, or cavalry in fantasy settings.
The Siege Unit — A heavy, armored monster for taking down buildings and other siege units. Either vulnerable to masses of lighter enemy troops, specialized counters, or equivalent enemy siege units. Usually a heavy tank or mech in modern and future settings. Siege units are often slow-moving and expensive.
The Anti-Something Vehicle — Like the infantry equivalent, only bigger, tougher, and can usually dish out more damage. These usually come in Anti-Infantry or Anti-Air.
The Artillery — Long-range equivalent to the siege unit, the artillery unit can't hit anything too close to it, has splash damage, and is usually under-armored. Its disadvantages are often balanced out by its massive attack power and long range.
The Elite Unit — A souped-up basic unit with a higher price tag, heavier armor, greater attack power, and/or higher speed for units of its type. These have a danger of getting spammed late in the game.
The Naval Unit — Typically works like floating artillery, but is well-armored and has access to seas or other regions that standard ground units cannot go. Also available in Submarine, Anti-Submarine, Anti-Air, and sometimes Aircraft Carrier. In space settings where spaceships take the place of ground units, this archetype does not apply.
The Air Unit — The air unit is often faster than ground units and mildly or not affected by terrain. In some cases it fights like an attack helicopter, hovering in one place to attack, even when it isn't one. In other cases, it has to return to a special building after every attack to rearm and/or repair. Good against anything that can't hit it back. Although, realistically, it shouldn't apply to spaceship-based games, this unit archetype is present anyway in the form of squadrons of small and maneuverable Space Fighters and similar strike craft.
The Transport — Picks up other units and carries them around, and is almost always an air or sea unit, although modern/futuristic games will often have land-bound APCs as well. Useful for transporting ground units over long distances, around defenses, or across inaccessible regions. Is often moderately fast and has little or no combat ability in itself.
The Hero Unit — A really powerful unit that more or less represents the player on the battlefield or a major character in the game's story. It usually has spells or abilities that can affect a small army at once. If nothing else has RPG Elements, it often will, with XP gain from kills and experience levels, and more abilities becoming available at higher levels.
The Super Unit — A large superpowered unit almost capable of taking out an army or base all by itself. These are usually expensive as hell to build (and usually slow, too), so if you allow your opponent to build several of these, you probably deserve what's about to happen (some games limit the player to only building one at a time). Awesome, but Impractical, bringing one of these into a fight often gets a satisfying Oh Crap from your opponent. Losing one of these returns the favor.
A critical part of all RTS games is the base-building and Resource Gathering element, which is also conducted very rapidly. The basic archetypes of an RTS base are:
The Command/Construction Center — Either constructs buildings itself (especially if in the Command & Conquer series) or builds Peon units.
The Resource Building — A building that stores or generates resources, and creates/accepts Peons, Workers, or Harvesters.
The Barracks — Builds the basic infantry units and sometimes handles unit-specific upgrades.
The Tech Building — A building whose main purpose is to advance the Tech Tree and upgrade units. Usually has to be built before you can build...
The Stable/Vehicle Factory — Builds scout, siege, and artillery units.
The Naval Building — Builds and repairs naval units.
The Advanced Tech Building — Like the first one, but comes later in the game, or is expanded from the original. Needed for accessing even more advanced stuff.
The Aircraft Factory — Builds (and upgrades) air units.
Defense Turrets — A small building with a mounted weapon. Usually varies between anti-infanty, anti-armor, and anti-air.
What order you contruct buildings in, and how quickly you do so, is often the most important factor in deciding the outcome, to the chagrin of some players.
The genre also possesses a number of aesthetic conceits, so typical to its presentation that they often confuse even the games' creators (Note: the first three are probably inherited from tabletop wargames):
Units are rarely depicted to scale, not even with each other. Buildings are shown about as high as mountains, vehicles tower over the structures that constructed them, and infantry are nearly as large as the vehicle that can carry ten of them.
A single "unit" onscreen rarely represents what it appears as. Rather, the single soldier is supposed to represent anything from a squad to an entire company, a hero unit usually includes their personal honor guard, and even a solitary building is probably symbolic of an entire complex.
In order to make things manageable in realtime, little details like ammunition, supply, noncombat casualties, fatigue, communication, and maintenance are conveniently swept under the rug.
Enhancing one's forces with better equipment and training makes perfect sense, but while a few titles do use plausible terminology for requisitioning new gear, training specialists, retooling factories and deploying the finished products to active forces, most use the peculiar choice of "research" for the process. This gives players the surreal image of scientists doing research on the front lines, making miraculous breakthroughs almost immediately, and then promptly forgetting whatever it was, every single level.
Pathfinding isn't necessarily especially weak in RTSs, but in other genres the player is usually too busy running along a linear path thwarting pre-scripted ambushes to notice. Not so in an RTS. Since RTSs take place in enormous, twisty campaigns and the player is constantly shepherding large numbers of units with a perfect view of their incompetence, the tendency of unit AI to unnecessarily bumble around the map in response to choke points, and clump together when attacking, becomes all too glaringly obvious.
Refinements of the genre formula over the years:
Addition of high ground bonuses for range and vision. Bonus damage for flanking enemy troops.
Organization of small units, like infantry and fighter craft, into squads or wings that act as one unit.
Terrain bonuses, such as cover and concealment.
Aircraft that act like aircraft, i.e. take off, perform mission, land again.
Elements from Real Time Tactics games, in which there is no base building or unit construction and scale is strictly squad-level, units are acquired at the start of each level.
The X-Universe series. note First person 4X space simulator with RTS elements. Once you own multiple ships, you can command them around the universe like in a RTS. Ship construction requires you to build factories and mines to get resources, and wait as the ship is assembled.