Tank

A French Renault FT-17 Tank, the first tank to have the rough layout that would be the norm for tanks (Crew in the front, top mounted 360 degree turret for main gun, engine in back)

A tank is a tracked, armored combat vehicle. The term is often limited to vehicles intended for direct combat, as opposed to e.g. self-propelled artillery (which stay to the rear) or armored personnel carriers and infantry fighting vehicles (which are on the front line but are primarily tasked with carrying soldiers as opposed to fighting directly). Their invention revolutionized warfare in the 20th century, and any wargame set in or after that time period (or in alternate universes with similar or more advanced technology levels) will have plenty of attention devoted to them -- or to whatever made them obsolete, as in e.g. BattleTech.

The idea of an armored fighting vehicle dates back at least to Leonardo da Vinci and H. G. Wells, but the modern tank was invented shortly before World War I, and was then spurred to production by the war itself. When the war on the Western Front got bogged down in trenches, the British Royal Navy had the idea to use tracked, armored vehicles with guns to break the stalemate. The name "tank" became attached to the vehicle as a codename to disguise the purpose of the large metal bodies being built. After the first tanks rolled onto the battlefield, other countries called them "battle wagons", "armors", "assault vehicles", and other more descriptive names, but the Anglosphere was stuck with calling them "tanks". (Interestingly, the original British Tank, which looked like a Leman Russ Battle Tank but without the turret, was called a "Little Willy".)

Tanks were built with pretty much any set of features you could imagine, but over time, the militaries of the world settled on several key features:

1. A large-caliber primary cannon for destroying enemy vehicles and defenses. This is partially why the Navy was the first branch of the British military to design and produce tanks during WWI, but mostly because the British army Derped out and did not see the value of tanks so the navy was forced to start development.
2. A turret to house the tank gun, to allow the tank to shoot at targets without having to pivot the entire vehicle. The French had this one figured out by 1918, and some tanks developed during the interwar period actually had more than one (though this proved impractical). Strictly speaking this one is not essential as the German Stug proves, and indeed the lack of a turret does have some advantages thanks to how it lowers the profile, but the advantages of a turret are strong enough to render the befits offered moot. A turret less tank is only really useful if you don't the money to make a turreted tank, or you don't the technology to mount a gun as big as you want in the turret.
3. A sloped, heavily-armored front face to absorb attacks, including those from enemy tanks. This in turn spurred the development of tank destroyers with even bigger guns at the cost of mobility, armour, or overall tactical flexibility (e.g. an inability to rotate their guns more than a few degrees to either side). It also led to the development of anti-tank weapons like missiles and mines specialized to attack more vulnerable rear, underside, and top armor. However Some times designers choose not to armor there tanks. If the thickest armor you can put on a tank is going to get penetrated anyway then your best bet is not to armor it at all and focus on speed to make sure you don't get hit to begin with. This was a very popular design during the cold war particularly by the French after the invention of high powered HEAT ammo but before Chobham armour, which is very good at stopping HEAT shells.
4. Tracks with a profile as low as possible while meeting all-terrain mobility needs. A stopped tank is a dead tank, and running the tracks over the top of the body is begging for a mobility kill, no matter how cool the British Mark I looked. (Though to be fair to the Mark I, it needed its high tracks to cross trenches, and since it came first, there weren't really any weapons that could take advantage of its exposed tracks at the time.)
5. A radio! It can not be understated how important a radio became to tanks. In both the battle for France and the early Operation Barbarossa the German tanks were under gunned and under armored compared to their opponents, but thanks to their radios, they were able to outmaneuver the enemy and take them apart. Radios also became important to tanks because, well, tanks are loud, and it's the only way for the crew to talk to each other without going hoarse yelling at each other.

Since Games Workshop seems to think that tank development in the Warhammer 40,000 universe stopped somewhere between the world wars, most of the iconic fighting vehicles of the Imperium lack some of these features. We can quibble that some of those vehicles are not really tanks but armored personnel carriers (e.g. the Land Raider) or other specialized classes of armored fighting vehicles, but GW's design team has a serious problem with looping their tracks all the way up and over the chassis for that 1918 flavor.

Types of Tanks

Whether its real or fantasy, tanks are classified from their weight and/or armament profile. Here are the common ones:

• Armored Personnel Carrier - APCs are light vehicles designed to carry infantry and not much else. They're usually given a heavy machine gun to support the infantry they're carrying into battle and to defend itself, and not much else. Unlike IFVs, APCs are not expected to fight on the front due to their lackluster protection and armaments.

• Artillery Tank - Tanks armed with artillery weapons, designed to bomb the enemy back into the stone age, ranging from howitzers, mortars, or missile systems. With that said, arty tanks tend to stay in back where they can do their thing. Not to mention that artillery pieces have a minimum range where they can drop their payload, thus, the tank needs to put some distance between them and their target so that they can be in effective range.

• Anti-Aircraft Tank - Tanks armed with weaponry designed to shoot aircraft out of the sky, ranging from flak guns to AA missiles.

• Infantry Fighting Vehicle - Known as IFVs, these tanks are capable of transporting infantry forces, while being armored and armed enough to be of support to the field, unlike light tanks. Unlike APCs, IFVs can be expected to stand up to enemy armor, while being able to transport infantry troops. Perhaps one of the more popular examples of these in Fantasy is the Space Marine Land Raider, which packs as much firepower as a front-line tank and can transport infantry at the same time.

• Light Tank - These are lightly armored tanks who sacrifice armor and firepower for maneuverability. They are not meant to be front-line combat tanks, as their armaments are usually too underpowered to go against heavier vehicles, rather they're usually relegated to reconnaissance duties and infantry support.

• Main Battle Tank - The catch-all term for any tank purely designed for front-line combat, MBTs are tanks designed for slugging it out with other tanks and generally being the armored spearhead of an assault force. Generally armed with a heavy cannon designed to knock out enemy armor and armored enough to take a beating from the opposition, all the while still capable of moving at a fairly rapid pace to keep up with the main force. Such real-world examples include the American M1 and the Russian T90, while in fantasy settings, these include the Predator Tank and the Hammerhead.

• Tank Destroyer - A specialist tank designed for one thing in mind: knock out other tanks and not much else, its essentially a metal box with a gun. Tank destroyers were made as a cost-effective way to destroy heavy armor, as MBTs were more expensive to produce and maintain due to the wider range of equipment, heavier armor, and larger fuel consumption. Thus, designers basically reused the same heavy gun used by the MBT, stripped off the armor, and usually fitted a lighter engine to reduce fuel consumption (as lighter armor meant the tank needed less power to get around).

• Super Heavy Tank - Superheavies were conceived in WW2, basically as a mini fortress on tracks, armed with a giant cannon and armor plating so heavy, you'd mistake it for a fortress. While some prototypes were fleshed out, none were put into service because they were too impractical; they were often too heavy to be supported by most roads, they moved too slow to be a threat while it consumed petrol like crazy as they usually needed a very powerful engine to even move at a snail's pace. Still, because fantasy is fantasy, that didn't stop writers from including such weapons in the arsenal of their armies, just to show how powerful they are. One of the more popular examples of superheavies is the BEEEIINBLADE, which is armed with no less than TWO main guns 9 other sponsor weapons.

Console RPG Character Role

In many role-playing video games, particularly the online ones, the term "tank" has also arisen to describe a character whose primary purpose is redirect all damage from enemies to himself. This was one of the primary purpose of actual tanks aswell; tanks, being as armored and threatening, are suppose to get most of the enemy's attention while the squishier units like infantry and light vehicles move into advantageous positions to deal more damage, without the threat of serious retaliation.

See, many enemies in RPGs have way too much health, deal way too much damage for most classes to withstand, and fights with them are unlikely to be decided in one round unless they're uncharacteristically vulnerable to save-or-die rays (which almost never happens). This is the usual case for boss-type, rare, and elite enemies.

Furthermore, many of the classes that are best at dealing damage (assassin and wizard types, for example) often have very little survivability when it comes to being punched in the face, in order to balance out classes (If a class can both tank damage and deal high damage at the same time, they either render other classes redundant or can do neither as well as a dedicated tank or damage dealer).

Thus, demand is created for a character whose job is to redirect enemies' aggression away from the squishy members of the party and towards them instead, usually using their mastery of mind-control, irritating sound effects, imposition of dangerous effects for attacking anyone else, or simply cutting insults and rude gestures which draw attention to themselves. They also tend to have abilities that help them in resisting, mitigating, avoiding, or regenerating from some of the damage they suffer (and on occasion act as a secondary damage dealer). In most cases, tanks are also often reliant on healer classes aswell to keep them alive while they do their thing, as enemies that require tanks can usually deplete a good chunk of their health in a few attacks. Making sure that chunk is constantly restored is required to make sure they can keep at it.

4th Edition Dungeons and Dragons refers to this role as the "defender," while Dawn of War 2 vets will recognize it as the "Tarkus", and later the "Diomedes."