I do actually program professionally.
Apps are going to be what we used to call 'Lite' versions of a tool. A subset of your product is broken out and provided as a Lite version so that users can see what its all about. In the case of Photoshop, it has a lot of basic filters most people commonly use and the ability to do some fun stuff with a picture. In essence, its marketing Photoshop to millions of users(and their friends who see them using the functionality), and by extension the CS suite of Adobe products.
If you want the full-blown tool, you go through the normal web channels of either buying the whole thing online or in a store.. or going through their subscription service. Expect to pay quite a bit.. whether over-time or upfront. Full product, you pay the big bux.
As per the technicalities of Apps vs Applications... Apps are like Applications wrapped in a very tight condom. Everything they do must be stated up front to the user, and if its special functionality like say access to the webcam, you ask very nicely with a confirmation box, and you can't really go offroading with them into the really esoteric portions of a system like you can with an Application. Its also tends to be slower than compiled native code.
That said, the backend of Windows as it currently sits Microsoft has nailed right to the wall. A lot of this stuff has been going in over years and nobody has really paid that much attention to it, especially if you programmed out of a traditional Windows Forms Application(like.. most developers).
With the App side of it, there is no ignoring it as you have no other choice but to dive head first into XAML for the front end(RT doesn't understand Forms). What they've managed to do is set it up so honestly, you can build both an App or an Application with essentially the same code. Its simply a different project using the same code and being compiled differently. There are some exceptions(like displaying Vector Graphics), but you should be able to make an interface library for RT and one for the Win Application that use the same calls but backend to their different platforms, one for each Project.
As such, whats good for one is good for the other.. like.. oh.. touch and pen. That stuff is native and baked right into the controls. I can display text, run the program, and immediately be able to pinch to zoom, scroll it with touch, whatever... all with not having to not write any touch-specific code for it.. it just works. Oh, and all previous libraries you've been writing in C#, VB, or C++ in previous years? That all works, either App or Application.
Apple should frankly be scared, to be quiet honest. If Development Enviornments were the Space Race, Microsoft just landed on the moon and Apple is still launching Mercury rockets.
I'm working my way through the Petzold book now(Programming Windows Edition 6). I highly suggest it for anybody who wants to get their footing and learn their way around the system. Most programs are very easy to understand and less than a page or so. Visual Studio 2012 Express for Windows 8 costs you nothing.