Web Servers are powerful computers that have extremely large hard drives, or an array of hard drives, that have been set up by a hosting company, usually in a facility called a datacenter. A web server's only purpose is to store websites until someone wants to visit them, and then to quickly respond to the visitor's browser with the site's contents.
A server should be able to respond to at least several dozen requests at a time, if not hundreds. If depends on many factors, including server hardware and how popular the sites are. But even the most expensive server hardware isn't going to be very useful without the software to actually get things done. The key software programs of a web server are also themselves called servers, or daemons.
Most servers used for web hosting have at least four kinds of software running at all times:
1) An HTTP or web server to provide the websites
2) An FTP server for uploading files
3) An email server or two
4) A database server for storing information essential to the operation of a website)