Complex sentence: A World Heritage Site is a landmark or area which is recognized by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as having cultural, historical, scientific or other form of significance, and is legally protected by international treaties.
Split into simpler sentences: A World Heritage Site is a landmark or area which has been officially recognized by the United Nations, specifically by the United Nations Educational, Scientific and Cultural Organization (UNESCO). Sites are selected on the basis of having cultural, historical, scientific or some other form of significance, and they are legally protected by international treaties.