The Warhammer Army books include several different types of characters, be they Wizards or Heroes. However, we can easily imagine all kinds of wild individuals with different characteristic values from those given in the army lists. The characters described above are 'standard' types. They are typically representative of their race, and of the sort of Heroes and Wizards you can expect to find in a Warhammer army. Variant types of character, those differing from the standard types, are referred to as 'special characters'.
Special characters are the Warhammer equivalent of famous warleaders and mages, individuals renowned amongst their own kind and abhorred by their enemies. For example, they can be the mighty leaders of nations, such as the Emperor Karl Franz of the Empire, Orc and Goblin Warlords such as Grom the Paunch of Misty Mountain, Malekith the Witch King of the Dark Elves, Hotek the renegade Priest of Vaul and many more besides. There are also important military leaders and high ranking nobles, such as Bretonnian Dukes, Elector Counts from the Empire, and devious Skaven Seers. All of these special characters are quite different from the standard types. Some are better fighters than others, but some are better leaders, while many carry specific magical weapons or ride large monsters.
Numerous special characters are described in the Warhammer Army books and other Warhammer supplements and, of course, players can get together and make up their own if they wish. After all, who can resist the temptation to create a mighty leader of armies in their own image, to paint and perhaps even model him to suit their heroic vision, to name and invent a past for their character, and to fight him in battle after battle!