A URL ending in a slash is for a folder or the index page of a folder.
URLs for pages do not end with a slash.
URLs for pages can have an extension or can be extensionless.
Images and media files (nearly) always require an extension.
If you use URL rewriting, extensionless URLs are best as you can then detect that any URL without a period needs to be rewritten to fetch the content from your dynamic script. The RegEx pattern matching is very simple.
This also completely avoids those inefficient -f and -d server filesystem "exists" checks that slow so many dynamic sites down.
URLs ending in .html will not be rewritten and the server will fetch the content from the matching static .html file. Extensionless URLs get rewritten to the dynamic script.
Simples! See also: [webmasterworld.com...]