There is roughly half a dozen nations in war thunder all with bascially the same vehicle types.

These nations are
America
Germany
Russia
Britain
Japan
China
France
Italy
And the newest nation
Sweden

Now there is no real best of worse nation all nations have both good and bad vehicles and typically overall have their own strengths and weaknesses so really it just comes down to personal preference.

Every nation has at least ground forces and aircraft dubbed "aviation" now nearly all nations have at least one helicopter but they are not counted under the avaiation tab and under their own tab.

Most nations also have a naval tech tree(what a tech tree is I'll get to soon) which is the naval aspect.

Virtually all tabs have their vehicles divided into tech trees which are a system of whatever somewhat resembling branches and so forth and to unlock I'm vehicle you must unlock the one before it and so forth.

Not all as if a nation only has a couple vehicles or even one vehicle alone then a tech tree will likely not be present.

Okay so the types of vehicles

Are
Tanks/ground vehicles
Aircraft
Helicopters
Ships

But within each of them there are various sub types of vehicles

In ground forces there are

Light tanks-generally some of the most quick and agile tanks around their light weight typically smaller size and usually lower profiles and the ability to recon and scout the area for their team often make them effective scouting vehicles and supporting units but in some cases can by used as direct assault vehicles their main pros include their light weight typically having a speed and maneuverability advantage and generally have quick reloading guns and at high levels can fire anti tank missiles which can be devastating weapons agasint enemy tanks. Their main cons however is that their light weight also me and they have light armor they are often easy to take out and have armor that's usually easy to pierce while some exceptions apply as some light tanks are actually decently well armored and while they have quick reload times their guns often lack in pure strengths but again some exceptions apply so overall their quick and nimble vehicles (as not all vehicles that are considered "light tanks" are actually more or less tanks) that make up for their fragility with speed and maneuverability.

Medium tanks-typically the bridge between heavy and light tanks medium tanks often fill the roll of a back of all trades master of none(but sometimes some) ground units they typically have more balanced stats between speed armor and firepower some medium tanks however can be very formidable units to face in the battle field as for the genuine pros and cons if often up to the nation's themselves as every nation has a somewhat different style of medium tank.

Heavy tank- the juggernauts of the battle field heavy tanks don't appear until you've advanced up the lineup a but in most cases from most nations and are infamous for their typically thick armor and very powerful guns they are often the best units for pardon the pun but "tanking" hits as they often have some of the best armor for their ranking around this heavy armor and large caliber guns do have a few drawbacks however they typically have slower rates of fire overall and their heavy armor often makes them heavy which results in typically lower speeds and are less agile in average then most other tanks  which can place them at a disadvantage at close quarters.

Tank destroyers- special variants of tanks or special vehicles over all that often have a high caliber gun for their ranking on a tank like chassis most tank destroyers and some go by the name "SPG" or "self propelled gun" they can be arguably split into two different groups one being thin armored small vehicles with exposed crew and a high caliber gun to massive hulking fully enclosed tank hunters with immensely powerful cannons most don't have the abilityto act as a regular tank however and as such are often best used as a sniping unit picking off enemy players from afar while some have relatively thick armor some have very quick reloads while some have long reloads but high power but a few tank destroyers can act as fully functioning tanks with the ability to rotate their gun 180 degrees but at lower ranks most tank destroyers are often light armored but with again very strong guns and at high rankings they can go by the name "anti tank missile carrier" which is a tank like vehicle that carries anti tank rockets and missiles but are still tank destroyers nonetheless. But most you encounter will often be lightly armored for their ranking.

SPAA-SPAA's or Self Propelled Anti Air
Are vehicles that have their primary focus in anti air firepower and taking down enemy aircraft and as such they often have machine guns and typically low caliber cannons but some have modified quicker firing tank cannons and at tip tiers some even have lock in surface to sir missile launchers that fire SAM(surface to air) missiles that are also fire and forget meaning you don't have to manually guide them to the target. Most SPAA's however are not very effective agasint tanks(*laughs in OTOMATIC*) and are typically best used away from the front line as they have some of the longest combat ranges in the game in order to deal with enemy bombers fighters helicopters and ground attack aircraft most low teir SPAA's are thin armored small vehicles with exposed crew while some higher teir SPAA's are modified tanks with different weapons resulting in much better armor and fully enclosed crews. But most typically have very light armor and can be taken out with ease if caught off guard.

In aircraft terms we have

Fighters- the faster and most maneuverable aircraft in the sky their typical maim task is to combat enemy fighters and aircraft but can be used to assist ground troops as such they are typically the most quick and agile aircraft in the game as well as some of the fastest without rockets or bombs and with most being armed with just machine guns or cannons they may not pose much threat to a main battle tanks but they can pose a massive threat to any vehicle with exposed crews and thin armor.

Attacker- attackers are planes typically similair to fighters except they feature better armor for a plane at least higher caliber guns and fsir better anti ground weapons this increase in firepower does often make them somewhat slower and less agile then fighters but most also carry a gunner mounted on the back of the plane which provides soem extra defence and most carry rockets or bombs to engage ground forces a few are truly agile enough to keep up with a fighter in a dogfight (the term used for at least two aircraft fighting agasint each other in the air) also their bombs and rockets and even cannons can pose a major threat to any ground units.

Bomber-bombers are the largest and heaviest aircraft in the sky typically having at least two engines with the most having four they carry payloads of bombs to drop and strike at ground units also often carrying mounted turrets in various places for defense agsisnt enemy aircraft. The bombers playstyle carries nation by nation but what is universal is that bombers especially the larger heavy bombers are large targets slow and heavy and can easily be out ran and out maneuvered by even attackers and heavy fighters(fighters with typically two engines instead of one) and can be very vulnerable to enemy fire but their bomb loads can be devastating to enemy forces.

All helicopters have the same classification "Helicopter"

Boats don't have many types to them.

There are however.

Motor torpedo boats/patrol boats-typically small fast and lightly armored and sometimes lightly armed ships they are among the most common ships you see at lower naval ranks armed typically with machine gun turrets maybe a few autocannons turrets torpedo launchers and from time to time depth charge launchers(a launcher that drops explosives under water to combat submarines ironically no official submarines exist in war thunder as of yet) the turrets are best used agasint enemy patrol boats and low flying aircraft while the torpedo launchers are extremely effective agasint enemy ships.

While very quick and agile most patrol and torpedo boats are very fragile and can be taken out by strafing fighters and enemy machine gun fire from enemy patrol boats.

Hydrofoil boat-the least common boat class in the game currently only Japan has a hydrofoil very simailir in role to a patrol boat except even faster typically has light armor and light armament but great speed and manuverability.

Destroyers-one of the more common ship types throughout the naval ranks destroyers are typically quick versatile ships with various weapons at their disposal including main caliber guns for ship to shit combat and partially strikes various anti air guns that vary in numbers from each nations ship types and torpedo and depth charge launchers for anti ship and anti submarine usage while typically very effective agasint patrol boats and aircraft agasint higher caliber and bigger class ships their main gun turrets may prove ineffective at long range and destroyers while able to withstand glancing hits from enemy high caliber guns and are nearly fully immune to the small machine gun and autocannons turrets of a patrol boat a few direct hits from a high powered anti ship weapon such as a cruisers guns or torpedo will at least cripple the ship.

Light cruiser- a less common ship found at higher rankings they are slightly larger and typically slightly better armed then destroyers in terms of armor and firepower they often serve roles simlair to destroyers and can pose a major threat to destroyers caught off guard and patrol/torpedo boats some are not armed with anti ship torpedoes or depth charge launchers forcing them to rely on their main guns to take out enemy ships but these are typically older style warships that where made before world war 2.

Heavy cruiser- one of the more uncommon ship types in the game as they are the apex predators of the naval tech trees and the largest ships in the game (other then the NPC aircraft carriers currently in the game) while slower then other ship types they have the best armor and survivability out of all the ship types but that also goes with low maneuverability but great firepower the strongest have a multitude of anti air guns smaller caliber secondary cannons for anti ship combat heavy caliber main guns that can cripple a destroyer in a single barrage and a few nations even have their heavy cruisers equipped with torpedo and depth charge launchers increasing their effectiveness even more.

That's all the vehicles types next chapter I'll go over the ammo types currently in the game