What is SERVER ?

In a technical sense, a server is an instance of a computer program that accepts and responds to requests made by another program, known as a client. Less formally, any device that runs server software could be considered a server as well. Servers are used to manage network resources. For example, a user may setup a server to control access to a network, send/receive e-mail, manage print jobs, or host a website. So Server Serves data on User's request.

Is Server a Computer or Anything Else ?

Yes Server is a Computer. Your Personal Computer ( Desktop, Laptop ) can be act as a server. To transform your PC into server you will need a special software.
Main Hardware of a Server :-

Processor
RAM
Hard Disc
Network Connection
Special Operating System

Look it is similar to your PC. Then what is main difference between a PC and a Commercial Server?
The only one difference is "Quality". The quality of HARDWARE of Server is better than PC. Because a Server runs 24 hours per day.

Types of Server :

Application server : Hosts web apps (computer programs that run inside a web browser) allowing users in the network to run and use them, without having to install a copy on their own computers. Unlike what the name might imply, these servers need not be part of the world wide web; any local network would do.
Catalog Server : Maintains an index or table of contents of information that can be found across a large distributed network, such as computers, users, files shared on file servers, and web apps. Directory server sand name servers are examples of catalog servers.
File server : Shares files and folder, storage space to hold files and folders, or both, over a network
Game server : Enables several computers or gaming devices to play multiplayer games
Mail Server : Makes email communication possible in the same way that a post office makes snail mail communication possible
Media Server : Shares digital video or digital audio over a network through media streaming (transmitting content in a way that portions received can be watched or listened as they arrive, as opposed downloading a whole huge file and then using it)
Print Server : Shares one or more printers over a network, thus eliminating the hassle of physical access
Proxy Server : Acts as an intermediary between a client and a server, accepting incoming traffic from the client and sending it to the server. Reasons for doing so includes content control and filtering, improving traffic performance, preventing unauthorized network access or simply routing the traffic over a large and complex network.
Web Server : Hosts web pages. A web server is what makes world wide web possible. Each website has one or more web servers.

List of Software that can turn Your PC into Server:

A Web Server

XAMPP, WAMP : Web Server
PLEX : Media Server