What ARE their jobS? -- It means that they have different jobs from each other or each of them having more than one duty etc.
What is their job? -- It means that they are working in the same capacity.
For example, if you see a group of policeman....
What are their jobs? I would answer one is a lieutenant, one is a traffic cop and the other is a detective.
What is their job? They are all keeping the public safe.