What technology is used.
The ultimate answer to "What technology should I use?" is "It depends." The traditional metaphor of a tradesman and his toolbox full of tools applies here. Many technologies overlap and many factors need to be considered. If the application in question is a full-featured multiplayer online game, then you will most likely decide on a beefy server-side application that talks to a database server and passes on various types of data to a client application that will both render the data and do some local computation/execution of code. The results will then be sent back to the server and the cycle will be repeated. So, what flavors are available?
On the server side your options are vast. Nearly all (contemporary mainstream) computer languages can interact with HTTP servers and database servers. In close to 100% of the cases the web server hands a chunk of data off to a computer program that gets more data from a database server. It then massages the data and sends a new or modified chunk of data back to the web server, and possibly to the database server as well. These computer programs can be written in Perl, Python, PHP, ASP, C/C++, Java, C#, or a variety of other languages.
Weaved ever so tightly into these two choices are a myriad of other technologies: What operating system will be used on the server? What operating system will be required of the client? What web browsers should be targeted? Will the web server be Microsoft's IIS, Apache, or some other HTTP server? What database will be used? Will it be a RDMS or an OO database? What intermediate means will be used to communicate between your applications and the database? A proprietary means or something like ODBC, JDBC or the dozens of other common database libraries and standards? These decisions will determine whether you have Sun hardware running Linux with Apache, JBoss, MySQL, using Java for JSPs, EJBs, and other code, or, just as likely, AMD hardware running Microsoft Windows with IIS, Tomcat, Oracle, using C#, .NET, and ASP.
Some groups of technologies are a more natural fit together than others. For example, if you are already familiar with Java you might choose to match Java, JSP, Java Faces, EJBs, Java applets, JDBC, and Apache + Tomcat. Or, equally likely, Microsoft shops will use Microsoft servers with Visual Studio development tools and .NET and ActiveX technologies. Either is perfectly viable and more a choice between left and right or up and down than right or wrong.
The key point to get from this is that you need to spend some time surveying all of the various tools available to you, determining where they are tuned to be used, and deciding if you have a personal affinity for any of them over others.
Next: Roll your own or just make a movie?