Apace web server Overview:

The Apache HTTP Server developed by the Apache Software Foundation (http://www.apache.org/). It was first developed to work with Linux/Unix operating systems, but was later adapted to work under other systems, including Windows and Mac. The Apache binary running under UNIX is called HTTPd (HTTP daemon)

(books.google.com)The function of a web server is to deliver web pages on the request of clients using the Hypertext Transfer Protocol (HTTP). This means delivery of HTML documents and any additional content that may be included by a document, such as images, style sheets and scripts.

(tutsplus.com)A web server gets a URL, translates it to a filename and sends that file back over the internet, If for any reason, the web server was not able to process and complete the request, it instead returns an error message.

The Apache server offers a number of services for use, include: HTTP, port 80, SMTP, port 25, DNS for mapping domain names to their corresponding IP addresses, port 53, and FTP for uploading and downloading files, port 21.