As of 2018, there are 1092 UNESCO World Heritage Sites, which includes 845 cultural and 209 natural sites.
But, what is a UNESCO World Heritage Site?
UNESCO stands for the United Nations Educational, Scientific and Cultural Organization, and is a branch of the United Nations (UN) that recognizes, preserves and protects sites that are an important part of the cultural heritage of the world.
These sites — amongst other things — "represent a masterpiece of human creative genius; or exhibit important developments in architecture, technology, monumental arts, town-planning or landscape design; or bear a unique testimony to a cultural tradition or to a civilization which is living or which has disappeared; or are areas of exceptional natural beauty; or are outstanding examples representing major stages of earth's history." (from the UNESCO World Heritage website)
As someone who has been lucky enough to visit quite a few of these sites, I can attest to the incredible importance of an organization like this to not only protect these important relics of human history, but to allow them to be safe and accessible to visitors from all over the world.