What a tank is
A protected gun that can move under fire. Everything else is a variation.
The three-way deal
Every tank is a compromise among firepower, protection, and mobility. Add more armor and the engine, bridges, and transporters complain. Add a bigger gun and you need a bigger turret ring, which needs a bigger hull. Make it light and fast and a cheap missile can kill it. There is no free variable — only which one a nation is willing to starve.
Not an IFV, not a tank destroyer
An infantry fighting vehicle carries a squad and a lesser gun. A tank destroyer (historically) existed to kill tanks and often skipped the turret or the all-around armor. A main battle tank is supposed to do all three jobs of the old light/medium/heavy split: scout when it must, smash when it can, and survive the shot that would have killed a specialist.
Why tracks
Tracks spread weight. A 65-tonne Abrams on tracks can cross soil that would swallow a wheeled truck of a quarter the mass. They also let the hull yaw in place. The cost is noise, maintenance, and speed on roads — which is why nations still keep wheeled tank destroyers and reconnaissance vehicles beside the tracked fleet.
The crew is the system
Western MBTs usually keep four people: commander, gunner, loader, driver. Soviet and French designs often drop the loader for an autoloader and run a crew of three. Fewer people means a smaller turret — and one more task per survivor if someone is wounded. Crew training, not millimetres of steel, is what actually wins gunnery duels.