A little while back I described the idea of an "application model."
The application model is a key concept that keeps getting muddled and confused with partially overlapping concepts like languages (e.g., Paul Graham's discussions of "languages for smart people" vs. "languages for the masses") and platforms (a legacy platform vs. a "great new" platform, etc.)
So besides being a useful term for which I'm not sure there is an equivalent, what is the role of application models?
First, application models -- not languages or platforms or vendors -- argue, persuade, and sometimes win.
Whether it's Visual Basic (the original), or C++ and OpenGL, or Rails, the question is "How well does the application model solve the world's (ok, domain's) problems for the folks who actually do the solving?"
And the cool thing is that application models are very malleable in a way that languages and platforms aren't -- because the they include (well, really their complements or negations include) all kinds of APIs and libs that people cook up.
Huh? Like this: the application model for JSP/Servlet apps (including "model 2") stunk. So folks created all sorts of libs (Struts, Tapestry, etc.) and templating engines (FreeMarker, Velocity, etc.) in order to create a better-shaped "hole" -- a better application model for what were still fundamentally Java Servlet apps. Still a hassle. Nice underpinnings, great performance, needs some work. We got IoC containers and the application model became friendlier. For small, quick apps, Rails had a lot of appeal, so we got Groovy and Grails as options. In the .Net world, we have C# adopting elements of Ruby (yes, they're really Smalltalk or Lisp elements) and ASP.Net offering the "MVC Framework" which -- with LINQ -- creates a flavor of Rails.
There is no intellectual property violation going on here. Platforms and languages are adapting around good application models. Some of this adaptation is vendor driven, some is community driven, some involves new languages, some involves new tools, and some involves APIs. But the common thread is a desire to alter an application model in a particular direction.
Application models are what get mindshare and what make provide for the continuity of commercial application development patterns over time. More on this point later.
Before I get there, it's worth pointing out that the application model also bears heavily on the aspects of application development apart from core feature development. When we get into these aspects, non-functional requirements, or orthogonal (really non-collinear) concerns, the application model determines (or at least posits an optimal or easiest path for) integration. How the UI attaches, how app-level security is managed, how different services are integrated -- these all fall under the sway of the app model, and if we want to make these things better, the app model is what we should be talking about.
Over time, the adaptation of application models (by the all of the parties mentioned above) is the story of the evolution of commercial software. Commercial software evolution is often described as a sequence of machines, or OSes, or platforms, or languages. It most definitely is not these things, any more than it is a story of vendors fighting and dying and rising. The thread of continuity, and the real underlying story, is about people adapting application models, piece by piece, over the years.