It's short for (Web Application ARchive), it's a jar file used for deploying web applications written to work on the JVM.

It's a zip file following Java convention that can be dropped in any Java web application server (like Apache Tomcat).

Basically a lot of sites you go to have been deployed by dropping a .war file in a Java web application server. That technologically powers a gigantic part of the Internet (just look at SO or Quora or expert exchange or any TIOOBE index or any Java job offer: Java webapps are pretty much powering the Real-World [TM]).

Are you suggesting that SO - StackOverflow.com - runs on Java?

Having no idea what the software stack is, and only basing a guess on having read plenty of Joel's musings, I'm guessing a Microsoft technology. I'm sure someone has the goods, or Google could probably answer.

It's an ASP.net MVC app, if I recall correctly. The .NET world has a parallel mechanism to .war deployment, though.

I think he is suggesting that the number of java related questions on those sites indicates the language is widely used. Neither SO or quora run on java.