Real links are the fundamental economic relationship among economies. These links have been usually associated with international trade. When two countries trade among themselves or if they compete in the same foreign markets, a devaluation of the exchange rate in one country deteriorates the other country's competitive advantage. As a consequence, both countries will likely end up devaluing their currencies to re-balance their external sectors. Other types of real links, like foreign direct investment across countries, may also be present.
Political links are the political relationships among countries.
This link is much less stressed in the literature. One example of political link is the following. Therefore, crises tend to be clustered. A crisis in one country is followed by crises elsewhere. When fundamentals and commons shocks do not fully explain the relationship among countries, spillover effects have been attributed to herding behavior, either rational or irrational.