What is server side programming?
Server side programming is the process of developing scripting or programming code designed to be executed on the server. The actual code is run on the server and never seen by the user; only the HTML code that results from executing the server file can be seen by the website user.

Examples of server side programming languages or technologies:
Perhaps the most common examples of server-sided programming languages include PHP, ASP (language itself is usually VBScript), ASP.net, Perl and ColdFusion among others. These files reside on the web server and are executed by the web application server to generate HTML code that can then be displayed by the browser of a web user. These files have their own file extensions such as .php, .asp, .aspx, .pl, .cfm and so forth.

What's so great about server side programming?
The benefit of server programming is that it allows you to have dynamic websites that can always change. The PHP or ASP page you develop is processed on the server and the resulting HTML code is displayed by the user's web browser. Depending on the request to the server by the web browser, the time and date, the query string, the posted form data and/or the contents of the database, the processed web site result displayed by the browser could thus vary. So basically one PHP or ASP page can produce and display a seemingly infinite number of web pages.