Server - a computer for industrial use. Applications in such a computer a lot. But let me tell you about a Web server.
Web server - a set of hardware and software parts designed to perform well-defined tasks. This server receives a request from a user, we are with you, via HTTP, gives us the answer in the HTML-page with content embedded in it. On a HTML-page can be placed text, images and diverse media content.
The web server also helps automate the job site, allows us to work on the site, use your account and collects statistics on visits.
What is the server's response, and is before us on the screen, is the site. The site can contain many different news articles, commentaries, videos. For the site hierarchy and display the required information and monitors your web server.
Client that you use when accessing the server's probably a web browser (from the English. Browse - view) as a desktop computer or tablet, and smartphone. In addition, there are programs that communicate with the Web server without the usual for us browser. For example, a bank-client or antivirus.
Often with a web server running side by side and the mail server. Work the mail server is on a different communication protocol. Mail server allows us to exchange e-mails.