I don't think that desktop apps are going anywhere -- but web apps are going to look a lot more like desktop apps very very soon.

I think it's safe to say that applications written in dynamic languages (like Javascript) that communicate with the Internet regularly are going to become more common. And a lot of these apps are going to run in something like Chrome or XULRunner.

A "desktop app" is a pretty simple distinction. You go to your "desktop" and load the app. From there, the app can read/write the file system, talk to the Internet, read the keyboard and mouse, communicate with special hardware devices, etc. Web apps are going to be very much like this very soon. And then there will be no difference between a web app and a desktop app.