Hello Chen,
"Job" and "occupation" could be synonyms and refer to one's post of employment or the work one does to earn money.
"What is your job/occupation?" would mean the same in that case and the answer could be " I am a teacher " or " i am a banker" etc...
'Career' on the other hand refers to the progression of one's professional life that entails training, development of skills and promotions. It is a life long perspective of the job you are going to choose and the general progression of this professional life.
So you don't usually ask 'What is your career?" and get a definite answer referring to a current job. It is rather used in a context such as 'What is your ideal career?" and the answer could be " I like working with number or records " or " i am good with machines and tools." or " I would love to pursue a career diplomat."