Tanks are the main "character" in World of Tanks. They vary in Nations, Tiers, and Types. Each tank has its own strengths and weaknesses, but as you progress through the tiers, they will become much more powerful.
There are currently 9 nations in World of Tanks: America, Britain, Russia, Germany, France, Czech, Japan, China, and Sweden.
Tiers are the level of the tank. There are 10 tiers, and in World of Tanks, they are displayed as Roman numerals: I, II, III, IV, V, VI, VII, VIII, IX, X.
Types are the class of a tank, and generally describe the role of the tank in a match. There are 5 types: Light, Medium, Heavy, Tank Destroyer, and SPG or the artilleries.