A web server is a HTTP server that respond to HTTP request, generally returning a web page (HTML) (but it can serve any type of files).
The request is handled:
When developing locally, note that your browser may enforce strict permissions for reading files out of the local file system. If you use xhr request locally (including d3.json et al.), you must have a local web server (ie installed in your computer)
For example:
See Web - Amp (Apache, Mysql, Php)
An origin server stores or creates resources such as HTML files and images. See HTTP - Origin Server (Header Field)