"Crank" is a slang term for individuals infected with the Flare virus. This is the reason WICKED was formed. They believe they can cure the Flare by studying the brains of the test subjects, some of which are not immune. "The Gone" is another term used for a stage of viral progression in those infected. Cranks who have succeeded past the Gone cannot be cured.
In the film series, Cranks are depicted as zombie-like creatures with strange vine-like growths protruding from their bodies. The book series clearly defines the differences between Cranks and traditional zombies, however, as Cranks are still alive and have the ability to speak.