Tank classification
Tank classification can be done either by weight or by role. Up until and during World War II, there was a tendency to create different tank classes for different specialised roles, e.g. infantry protection. In the later half of World War II and after, this trend was reversed, the specialised roles largely disappeared, as did the distinction between medium and heavy tanks. Instead, tanks were designed to be able to handle most roles previously handled by different tank types and for some roles were replaced by different armoured vehicles. Most notably this occured with the light tank, which was largely abandonded for the armoured car or the infantry fighting vehicle in the reconnaissance role.
Classification by weight or function?
Classifying tanks across the almost 100 years of their existence is difficult.
Tanks are often referred to by weight-based classification, such as 'light', 'medium' or heavy'. Many types are also described by their tactical role, and these descriptions depends on the tactical doctrine of their army at the time. For instance 'cruiser' and 'infantry' tanks are British classifications of the 30s and 40s; 'breakthrough' and 'fast' are contemporaneous Russian types. 'Light', 'Medium' and 'Heavy' tank descriptions also imply tactical roles. Furthermore, expected weights for a given tank type vary over time; a medium tank of 1939 could weigh less than a light tank of 1945. The British Matilda infantry tank weighed as much as a German Panzer III or Panzer IV medium tank but due to its slow speed and heavy armour should arguably be classified as a heavy tank.
Tank Types
Tankette
A tankette was a small tank, with a crew of 2 (there were prototypes with one-man crew). It usually had no turret, or if it did, it was traversed by hand. It was armed with 1 or 2 machineguns, or rarely with 20mm gun. The "classic" design was British Carden-Loyd Mk.VI Tankette - many others were modelled after it. Tankettes were produced between about 1930 and 1939, but ceased soon afterwards due to limited usefulness and extreme vulnerability.
It could be argued that due to the demand for light, airmobile fighting vehicles, the tankette is making a comeback in the form of the Wiesel1 IFV (although it is called a weapons carrier)
Light tank
These tanks were designed for speed and to be able to take on infantry and light vehicles. While very common at the start of World War II they were graudually relegated to use as scouts and to strike vulnerable areas. Some, like the Tetrarch were small enough to be air-lifted to the battlefield. Sometimes called a cavalry tank.
Medium tank
Tanks with a good balance of firepower, mobility and protection. Able to engage infantry and also other tanks. German Panzer III, Panzer IV and Panther tanks were medium tanks of increasing sophistication and power, as were the American M3 Grant and M4 Sherman and Russian T-34.
Heavy tank
Designed to attack obstacles, create breakthroughs and engage enemy armoured formations. Prominent World War II examples were the German Tiger and Russian Josef Stalin. They featured very heavy armour and guns. After World War II the heavy tank concept evolved into the main battle tank.
Assault tank
A very heavily-armoured vehicle designed to directly attack heavy anti-tank emplacements, of limited tactical use otherwise. While the German Panzer IV was originally envisaged as something like this, World War II versions included the Sherman Jumbo, the Sturmmoerser Tiger and the Churchill AVRE.
Cruiser tank
A Cruiser tank is a tank that is designed to move fast and destroy enemy tanks. The idea came from the concept of "Fast Tanks" pioneered by Walter Christie.
They were used by the United Kingdom during World War II and were typically thinly armored and armed with with anti-tank guns that could not effectively combat infantry or towed anti-tank weaponry. Cruiser tanks were designed to complement infantry tanks, exploiting gains made by the latter to breakthrough enemy lines and assault from the rear. In practice they largely proved to be less effective than the German tanks they opposed.
The early Cruiser tanks (marks A9 and A10) were very lightly-armoured and their only redeeming feature was the useful 2pdr gun, which was perfectly capable against German tanks of the French and Greek campaigns. Cruiser squadrons were replaced either by the Crusader (a more heavily armoured version) or the American M3 Stuart light tank - referred to by its crews as the 'Honey'. British cruiser tank units were then in 1942-1944 equipped with American M4 Sherman tanks. In 1944 they wre partially converted back to British-made Cromwell cruiser tanks which were much faster than the Sherman but less well-armoured. The final cruiser tank was the Comet, introduced in late 1944, which was an extremely effective medium tank on a part with the German Panther.
Infantry tank
The idea for this tank was developed during World War I by the British and French. The infantry tank was designed to be slow (moving at the same pace as running infantry) and heavily armored. Its main purpose would have been to clear the battlefield of obstacles, take out enemy soldiers, and protect the infantry on their advance through and into enemy lines.
The best-known infantry tanks were the Matildas of World War II. Their armour and anti-tank firepower was sufficient to take on German Panzer IIIs. However, their anti-infantry weaponry was often restricted to a machine-gun, with no high-explosive rounds for their main gun being available. Later British infantry tanks were more akin to medium tanks and were overshadowed by the American Lend-Lease tanks that they were used alongside.
Specialist Tank
Tanks have often been modified for special purposes. Mostly they are there to provide an armoured capability to work alongside combat engineers. Such tanks include tanks with flails or plows to clear mines or flame tanks mounted with flamethrowers.
The other common modification is the amphibious tank, such as the Sherman Duplex Drive. These designs are modified with waterproofing and propulsion systems to be able to traverse open water. Their most notable usage was on D-Day.
Main battle tank
The MBT is a heavy modern tank, designed to be the backbone and the main tool of the modern armored forces. It is armed and armored to face all kinds of threat - tanks, anti-tank missiles, infantry and stationary fortifications. The MBT can be classified as an heavy tank, as modern MBTs weight at least 60-70 tons and are armed with 120mm main gun.