A web server is simply a computer program that dispenses web pages as they are requested. The machine the program runs on is usually also called a server, and the two references are interchangeable in everyday conversation.
Every web page on the internet also has a unique address, called a Uniform Resource Locator, or URL.
When a user types that URL into a web browser, the machine the browser is running on sends a request to the IP address of the machine running the web server for that page, requesting that all the content found there be sent back. Once the web server receives that request, it sends the page content back to the IP address of the computer asking for it. The web browser then translates that content into all of the text, pictures, links, videos, etc. that so many web pages contain.
The data being transferred back and forth during these exchanges must conform to a specific protocol, called Hypertext Transfer Protocol (HTTP), to ensure that all web pages and servers communicate with one another efficiently and without error. The most common web server software today, that is, the program that actually does the receiving and answering of page requests, is Apache. This open-source software handles more than half of all websites in existence today - many developers of web-based applications and technologies use Apache as their default environment when designing new products. The second most common is Internet Information Services (IIS), released by Microsoft, which accounts for nearly 25% of all websites. Website Link