Prepare for a brief history lesson...
Going back to
WFRP 1st Edition (1980s), race and starting career (class) were determined randomly. This was a thematic choice that communicated that your lot in life was at the whim of the gods. However, after gaining experience, you could switch careers, eventually progressing (for example) from lowly Rat Catcher to Footpad to Highwayman to Outlaw Chief (an Advanced Career).
In
WFRP 3rd Edition, this philosophy is largely kept intact. You may select or randomize your race: human, dwarf, high elf or wood elf. You then draw three career cards and select one you like, or you can roll. Of course, the
GM could allow you to select your favorite career--
FFG's Rules Lawyers won't knock on your door.
Similar to WFRP 1e, 3e allows you to switch careers when you gain enough experience. This system is more flexible than in 1e. (2nd edition is more like 1st edition if you're wondering.)
Regarding customizing the character, you are given creation points to improve characteristics (Strength etc.) and select talents, skills, etc.
I haven't played it, I'm just looking at the rulebook, so hopefully someone can chime in and tell you what they think.
Happy gaming!