The official website of the World Heritage Committee is a great starting point to learn about World Heritage Sites. Their World Heritage List page has many handy links, including to World Heritage Sites in each country, a map view showing the location of each site, newly added sites, and sites listed as In Danger.
Each individual World Heritage Site has its own separate page, with multilingual information about the site. There’s a brief overview of the site itself; while further down the page you’ll find additional information about the site, its Integrity (e.g. how intact the site is), its authenticity (how much the site has been affected by restoration and modification), along with protection and management requirements.
There’s also some photos, any World Heritage-relevant news about the site, and some links to further reading. Each site page also has links to official maps of the site, outlining exactly what is and isn’t included within the boundary, plus any official UNESCO documentation relating to the site.