A web server is a server dedicated to hosting websites. A web server is tailored and optimized only to serve web sites for an intranet or the internet. A web server responds to HTTP requests. A web server may server raw text, an image, or some other form of document. A web server will also send error responses if necessary.
Apache has been the most popular web server on the internet since April of 1996 and has about 60% web servers running it.