A server is a computer that serves many kinds of information to a user or client machine. 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.
For servers and clients to talk to each other, they need to be connected to a network. They need to use the same communication protocol, a set way for machines to talk to other machines. It is 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.