C is for Career
Before your character decided to go adventuring, he had a day job. That job supplied him with pay (hopefully) and skills...some of which can translate into other lines of work, like adventuring and intrigue. This job is called the character's career. Enterprising characters may still be active in their career, adventuring on the nights and weekends, so to speak. Once a character has improved withing that career as much as possible, WarHammer lists other careers that are natural progressions. Some, called advanced careers, are much more talented than mere beginners and can have a much greater effect on the world around them.
When I first read about careers I thought they were pretty ingenious. They gave a background to characters that other early roleplaying games don't really give. I knew more about my character, how he was raised, and what his everyday skills were. This can also lead to interesting points like who, or what kind of people, the character has for contacts. Is his career rural or urban? These set the stage for adventure.
Careers add to the realism of the game, something some players really enjoy. Just that one distinction, a career, provides an amazing springboard. In my mind it is an iconic part of WarHammer.