A website is made up of one or more computer files for each web page. These files are stored on computers called hosts, that are directly connected to the internet. Generally hosts operate continuously and are constantly monitored for proper operation. Whenever someone wants to see one of your web pages,a request is sent to your host, which transmits the necessary files to the requestor. If your host is not operating, nobody can see your web pages.
Host computers on the internet are identified by long strings of digits which are decidedly unfriendly to remember. As such, part of the internet mechanisms allow names that people can remember, such as www.coke.com, or www.whitehouse.gov, or www.amazon.com.
There are special computers on the internet known as name servers, whose sole purpose is to provide the translation from these more user friendly names to the numeric identification of the corresponding host computer.
This translation mechanism has additional advantages. It allows a website to move from one computer to another, simply by changing the information tables in the name servers. Perhaps one computer is being overloaded, or physically replaced, or you found another hosting service that is cheaper or more reliable. The internet naming methods gives you the equivalent of one address for life.
Of course, there must be some method to make sure that the names that the assigned names are unique, and that changes are distributed to all the name servers together. This is the role of corporations called Internet Registrars, and they charge a small annual fee to maintain each unique name.
An internet browser, such as Internet Explorer or Firefox, are applications that run on your computer. When you want to see a web page, they send the requests on the internet for the needed information, assemble and interpret the files received, and display the results.
There is a standards organization that specifies the structure and options within these page files. However, these standards are constantly evolving, some websites are old, people do not all have current versions of their browser, so mixed levels of capabilities is normal. Further, no current browser supports the full standard. In order to gain a competitive advantage, individual browsers have also introduced and implemented their own ideas, which are not contained in the standard. To make things word, if a browser encounters something unexpected, it is ignored. As a result, there is no expectation that the display of a web page will be the same for all users.