I searched on Wikipedia , but i didn't understood the explanation.
But how come web server ?
I have worked on Tomcat server and having knowledge of J2EE framwork,
here a browser send request using a url, now this request hits the web container(Tomcat),
now web container creates req. and res. object and passes it to servlets deployed in webcontainer.
New engines like Node.js give it capabilities like threads and sockets, which means it can run like any server side language.