In net jargon, a server is a computer that serves many kinds of information to user or client machines. Usually a server will only do a few things for many clients. Every type of thing a server does is called a service. Services are used by other computers that are called clients. The relationship between client and server is called a client-server relationship. For example, Wikipedia has web servers which have a service for sending web pages over the Internet. Your client computer talks to Wikipedia's web page service to get web pages for you. A server can also host internet games, share files, and give access to peripheral equipment such as printers. In simple words, the individual computers are connected to some powerful computers called servers. These store files and information in the form of website. With an Internet connection, different users anywhere in the world can access these files.
So that servers and clients can talk to each other, they need to be connected to a network, and they need to use the same protocol. In networks, a protocol is a set way for machines to talk to other machines. It is just like a language. For example, the Wikipedia server runs the HTTP to send web sites to your computer, and your computer uses the HTTP Protocol to ask Wikipedia for pages.
Usually, servers are specially made to be more powerful and reliable, and they are much more expensive than normal computers. Sometimes, servers can be clustered, which means that lots of servers work together to do one service.
The server might slow down if there are too many people accessing the server at the same time, resulting in a high load. It might also be unable to take the high load, so it will automatically shut down.
Typical server operating systems are Linux, FreeBSD, NetBSD, and OpenBSD. Unlike other computers, a server often has no monitor, keyboard, or mouse. When a server doesn't have to do very much, server software can run on a computer that is also doing other things.
A web server is a type of server that is used to host websites. Examples of web server software include Apache or IIS. A web server can host one, or many, websites. The default port for a web server to listen to is port 80 (HTTP) or 443 (HTTPS).
Some web servers do other things than just serving a web page. For example, they may have something called SSI that makes building a website easier.
Web servers use services like CGI to let software on the server make web pages. Some of the programming languages that can use CGI are scripting languages like Perl, Python, PHP, or ASP. Some are compiled languages like C++ or Java.