Windows Store Apps are what was once called 'Metro' apps and now termed 'Modern' apps. They are essentially Microsoft's version of apps from iOS app store or Google Play store but focused on Windows 8 tablet and desktops (and as of today, the same apps you develop for these environments can also work on Windows Phone and XBOX One)

In the past if you were writing a client side or 'desktop' app for Windows you used C++ and MFC or a .NET language with Windows Forms or WPF (or lots of other options) to build that experience. Going forward Microsoft would prefer you build using this new model they are calling Windows Store apps. Along with that comes a set of rules and explicit declarations about what your app will do (much like we have on Android or iOS). This should in theory limit the number of crapware that exists, though that remains to be seen.

Jitter is a term we often use for a program that does the JIT work, JIT is an acronym for Just In Time which in context means Just In Time Compiler; it runs just before the code is needed. You have a program with 10K lines of code but the command you just sent the program only invokes 500 lines - a jitter or Just in time compiler will only convert the code that is actually needs to perform the operation you requested (thus performing faster).