A tank is an armoured fighting vehicle designed for front-line combat. Tanks have heavy firepower, strong armour, and good battlefield manoeuvrability provided by tracks and a powerful engine; usually their main armament is mounted in a turret. They are a mainstay of modern 20th and 21st century ground forces and a key part of combined arms combat.
Modern tanks are versatile mobile land weapon system platforms that have a mounted large-calibre cannon called tank gun in a rotating gun turret supplemented by mounted machine guns or other weapons such as anti-tank guided missiles or rockets. They have heavy vehicle armour which provides protection for the crew, the vehicle's weapons, and propulsion systems as well as provide operational mobility due to its use of tracks rather than wheels which allows the tank to move over rugged terrain and adverse conditions such as mud (and be positioned on the battlefield in advantageous locations). These features enable the tank to perform well in a variety of intense combat situations, simultaneously both offensively (with fire from their powerful tank gun) and defensively (due to their near invulnerability to common firearms and good resistance to heavier weapons, all while maintaining the mobility needed to exploit changing tactical situations). Fully integrating tanks into modern military forces spawned a new era of combat: armoured warfare.
There are classes of tanks: some being larger and very heavily armoured and with high calibre guns, while others are smaller, lightly armoured, and equipped with a smaller calibre and lighter gun. These smaller tanks move over terrain with speed and agility and can perform a reconnaissance role in addition to engaging enemy targets. The smaller faster tank would not normally engage in battle with a larger, heavily armoured tank, except during a surprise flanking manoeuvre.
ARTICLES SOURCES : https://en.wikipedia.org/wiki/Tank