The difference between the two sentences lies in the certainty and level of decision-making expressed.

1. "I am going to work abroad": This sentence indicates a more definite plan or intention. The person has made a decision and is expressing their intention to work abroad after graduating. It implies that they have already made arrangements or taken steps towards making it happen.

2. "I think I will work abroad": This sentence suggests that the person is considering working abroad but hasn't made a final decision yet. It shows a level of uncertainty or contemplation. The person is expressing their current thought or inclination, but it doesn't necessarily mean they have committed to working abroad after graduation. They may still be exploring other options or considering different factors before making a final decision.