/var/www/html: The actual web content, which by default only consists of the default Nginx page you saw earlier, is served out of the /var/www/html directory. This can be changed by altering Nginx configuration files.
/etc/nginx: The Nginx configuration directory. All of the Nginx configuration files reside here.
/etc/nginx/nginx.conf: The main Nginx configuration file. This can be modified to make changes to the Nginx global configuration.
/etc/nginx/sites-available/: The directory where per-site server blocks can be stored. Nginx will not use the configuration files found in this directory unless they are linked to the sites-enabled directory. Typically, all server block configuration is done in this directory, and then enabled by linking to the other directory.
/etc/nginx/sites-enabled/: The directory where enabled per-site server blocks are stored. Typically, these are created by linking to configuration files found in the sites-available directory.
/etc/nginx/snippets: This directory contains configuration fragments that can be included elsewhere in the Nginx configuration. Potentially repeatable configuration segments are good candidates for refactoring into snippets.